Savory Potato and Caramelized Fennel Gratin Bake

Quick Recipe Version (TL;DR)

• Yield: 6 servings
• Prep Time: 30 minutes
• Cook Time: 1 hour 15 minutes
• Total Time: 1 hour 45 minutes

Quick Ingredients

• 2 large fennel bulbs (about 1½ lb / 680 g), cored and thinly sliced
• 2 medium yellow onions, thinly sliced
• 3 tbsp olive oil, divided
• 1 tbsp unsalted butter
• ½ tsp sugar
• 2 lb (900 g) Yukon Gold potatoes, sliced 1/8 in (3 mm) thick
• 3 cloves garlic, minced
• 1½ tsp fresh thyme leaves (or ½ tsp dried)
• 1¼ tsp kosher salt, divided, plus more to taste
• ¼ tsp black pepper, plus extra for topping
• Pinch red pepper flakes (optional)
• 1 cup (240 ml) heavy cream
• ½ cup (120 ml) whole milk
• ½ cup (120 ml) low-sodium chicken or vegetable broth
• ½ tsp Dijon mustard
• ¼ tsp ground nutmeg
• 1½ cups (150 g) grated Gruyère cheese
• ½ cup (45 g) grated Parmesan cheese
• 1 tbsp olive oil or melted butter (for drizzling)
• Fennel fronds or parsley, chopped, for garnish

Do This

• 1. Heat oven to 375°F (190°C). Grease a 9×13-inch baking dish or 12-inch oven-safe skillet.
• 2. Caramelize fennel and onions in 2 tbsp olive oil and butter with ½ tsp sugar and a pinch of salt until deep golden, 20–25 minutes.
• 3. Whisk cream, milk, broth, Dijon, nutmeg, and ½ tsp salt. Toss potatoes with garlic, thyme, ¼ tsp pepper, remaining ¾ tsp salt, and red pepper flakes if using.
• 4. Layer half the fennel mixture, then half the potatoes (slightly overlapping). Repeat layers. Pour cream mixture evenly over the top, pressing potatoes down.
• 5. Cover tightly with foil and bake 45 minutes, until potatoes are mostly tender.
• 6. Uncover, top with Gruyère and Parmesan, drizzle 1 tbsp oil or melted butter, season with pepper. Bake 20–30 minutes more, until deeply golden and bubbling.
• 7. Rest 10–15 minutes so it sets. Garnish with chopped fennel fronds or parsley and a pinch of flaky salt, then serve warm.

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Why You’ll Love This Recipe

• Layered potatoes and slow-caramelized fennel create a dish that is both cozy and elegant.
• A rich Gruyère-Parmesan crust turns irresistibly golden and crispy in the oven.
• Simple, everyday ingredients come together in a way that feels restaurant-worthy.
• Perfect as a hearty side dish for roasted meats or as a vegetarian main with a green salad.

Grocery List

• Produce: Fennel bulbs, yellow onions, Yukon Gold potatoes, garlic, fresh thyme (or dried), fresh parsley or fennel fronds.
• Dairy: Unsalted butter, heavy cream, whole milk, Gruyère cheese, Parmesan cheese.
• Pantry: Olive oil, low-sodium chicken or vegetable broth, kosher salt, black pepper, red pepper flakes, sugar, Dijon mustard, ground nutmeg.

Full Ingredients

For the potato and fennel layers

• 2 large fennel bulbs (about 1½ lb / 680 g), stalks and fronds trimmed, bulbs cored and thinly sliced
• 2 medium yellow onions, thinly sliced (about 2 cups)
• 3 tbsp olive oil, divided
• 1 tbsp unsalted butter
• ½ tsp sugar
• 2 lb (900 g) Yukon Gold potatoes, scrubbed and thinly sliced 1/8 inch (3 mm) thick (peeling optional)
• 3 cloves garlic, minced
• 1½ tsp fresh thyme leaves, finely chopped (or ½ tsp dried thyme)
• 1¼ tsp kosher salt, divided, plus more to taste
• ¼ tsp freshly ground black pepper
• Pinch red pepper flakes (optional, for gentle heat)

For the creamy base

• 1 cup (240 ml) heavy cream
• ½ cup (120 ml) whole milk
• ½ cup (120 ml) low-sodium chicken or vegetable broth
• ½ tsp Dijon mustard
• ¼ tsp ground nutmeg (freshly grated if possible)
• ½ tsp kosher salt

For the aromatic cheese topping

• 1½ cups (150 g) coarsely grated Gruyère cheese
• ½ cup (45 g) finely grated Parmesan cheese
• 1 tbsp olive oil or melted unsalted butter, for drizzling
• Freshly ground black pepper, to taste

To finish and serve

• 1–2 tbsp chopped fennel fronds and/or fresh flat-leaf parsley
• Flaky sea salt, for finishing (optional)

Step-by-Step Instructions

Step 1: Preheat the oven and prepare the baking dish

Preheat your oven to 375°F (190°C) with a rack in the center. Lightly grease a 9×13-inch (23×33 cm) baking dish or a 12-inch oven-safe skillet or cast-iron pan with a little olive oil or butter to prevent sticking. Set aside.

As the oven heats, gather all your ingredients. This recipe moves more smoothly if everything is prepped before you start caramelizing the fennel and onions.

Step 2: Slice the fennel, onions, and potatoes

Trim the fennel bulbs by cutting off the stalks and fronds (reserve some fronds for garnish). Slice each bulb in half lengthwise, remove the tough core, then slice the fennel halves thinly (about 1/8–1/4 inch thick). Thin slices will soften and caramelize more evenly.

Peel the onions and slice them thinly into half-moons.

Scrub the potatoes (peel if you prefer a more refined look). Using a sharp knife or mandoline, slice the potatoes into 1/8-inch (3 mm) rounds. Try to keep them as even as possible so they cook at the same rate. Set the slices in a large bowl. Do not rinse them; you want the natural starch to help thicken the creamy sauce.

Step 3: Caramelize the fennel and onions

In a large skillet over medium heat, add 2 tablespoons olive oil and the 1 tablespoon butter. Once the butter has melted and is foaming, add the sliced fennel and onions. Sprinkle with about ¼ teaspoon of the kosher salt and the ½ teaspoon sugar, which helps them brown more evenly.

Cook, stirring occasionally, for 20–25 minutes, adjusting the heat between medium and medium-low as needed. The goal is to slowly soften and deeply caramelize the fennel and onions without burning. They should become golden-brown, jammy, and sweet. If they start to brown too fast, lower the heat and add a splash of water to loosen any browned bits. Taste and add a pinch more salt if needed.

Once caramelized, remove the skillet from the heat and set aside.

Step 4: Season the potatoes and mix the creamy base

To the bowl of sliced potatoes, add the minced garlic, thyme, remaining kosher salt (about 1 teaspoon, or to taste), ¼ teaspoon black pepper, and a pinch of red pepper flakes if you like a hint of warmth. Toss gently with clean hands or tongs, making sure the seasoning is evenly distributed over the slices.

In a separate bowl or large measuring jug, whisk together the heavy cream, whole milk, broth, Dijon mustard, nutmeg, and ½ teaspoon kosher salt. Taste the mixture; it should be pleasantly seasoned and slightly savory. This liquid will thicken as it bakes, becoming a rich sauce that coats the potatoes and fennel.

Step 5: Layer the potatoes and fennel

Spread half of the caramelized fennel and onions evenly over the bottom of the prepared baking dish or skillet.

Arrange half of the potato slices over the fennel in slightly overlapping layers, like shingles on a roof. This not only looks attractive but also helps the dish cook evenly and hold together when sliced.

Scatter the remaining caramelized fennel and onions over the potatoes, then finish with the final layer of potatoes, again slightly overlapping the slices.

Slowly pour the cream mixture evenly over the entire dish, tilting the pan gently to distribute it if necessary. Use your hands or a spatula to gently press down on the potatoes so the liquid seeps between the layers. You want most of the potatoes to be in contact with the creamy base, with just the very top layer peeking out.

Step 6: Bake covered until nearly tender

Cover the dish tightly with aluminum foil to trap steam. Place it on the center rack of the preheated oven and bake for 45 minutes, or until the potatoes are mostly tender when pierced with the tip of a knife. You should feel only slight resistance in the center.

At this stage, the potatoes will be cooked through enough to eat, but the top will still be pale. The next step adds the aromatic cheese topping and turns it golden and bubbly.

Step 7: Add the cheese topping and finish baking

Remove the baking dish from the oven and carefully discard the foil (watch for steam). Evenly sprinkle the top with the grated Gruyère and Parmesan. Drizzle the surface with 1 tablespoon olive oil or melted butter and finish with a few grinds of black pepper.

Return the dish to the oven, uncovered, and bake for an additional 20–30 minutes, or until the cheese is deeply golden, bubbling around the edges, and the potatoes are completely tender.

If you want extra browning on the cheese, you can switch the oven to broil for the last 2–3 minutes, watching very closely so it does not burn.

Remove from the oven and let the potato and fennel bake rest for 10–15 minutes. This resting time allows the creamy sauce to thicken and the layers to set, making the dish easier to slice and serve.

Just before serving, sprinkle with chopped fennel fronds and/or parsley, and a pinch of flaky sea salt if you like a little crunch on top.

Pro Tips

• Slice evenly: Use a mandoline if you have one for perfectly even potato slices that cook at the same rate and create beautiful, tidy layers.
• Low and slow caramelizing: Do not rush the fennel and onions. Proper caramelization adds a deep, sweet flavor that makes this dish special.
• Taste the cream mixture: The cream should taste slightly salty and flavorful before baking; that seasoning has to carry through all the potatoes.
• Let it rest: The bake will look a bit loose when it first comes out. Resting 10–15 minutes is the secret to neat slices and a silky, not runny, sauce.
• Use the right pan: A heavy ceramic dish or cast-iron skillet holds heat well and promotes even cooking and browning, especially around the edges.

Variations

• Herb-forward version: Swap thyme for a mix of fresh rosemary and sage, finely chopped. Add them to both the potatoes and the cream mixture for a more woodsy, aromatic flavor.
• Smoky and meaty: Stir 4–6 slices of crisp-cooked, chopped bacon or pancetta into the caramelized fennel and onions, and add a small handful of smoked cheese (such as smoked Gruyère or smoked cheddar) to the topping.
• Lighter, brothier bake: Replace half the cream with additional broth and use a little less cheese on top. The dish will be a bit less rich but still full of flavor.

Storage & Make-Ahead

Let leftovers cool to room temperature, then cover tightly and refrigerate for up to 3 days. Reheat individual portions in the microwave, or rewarm the whole dish, covered with foil, in a 350°F (175°C) oven until hot throughout, then uncover for the last 5–10 minutes to re-crisp the top.

To make ahead, you can fully assemble the dish up through Step 5 (with the cream poured over) up to 24 hours in advance. Cover tightly and refrigerate. When ready to bake, let it sit at room temperature for 20–30 minutes while the oven preheats, then proceed with baking as directed, adding a few extra minutes if needed since it will be starting from cold.

Nutrition (per serving)

Approximate values for one of 6 servings: about 420 calories, 24 g fat (of which about 13 g saturated), 40 g carbohydrates, 3 g fiber, 6 g sugars, 13 g protein, and around 650 mg sodium. Actual values will vary based on specific ingredients and brands used.