Creamy Potato, Leek, and Bacon Gratin Casserole

Quick Recipe Version (TL;DR)

• Yield: 6 generous servings
• Prep Time: 25 minutes
• Cook Time: 1 hour 10 minutes
• Total Time: 1 hour 35 minutes

Quick Ingredients

• 3 lb (1.4 kg) Yukon Gold potatoes, sliced 1/8 inch (3 mm) thick
• 3 medium leeks, white and light green parts, thinly sliced
• 8 oz (225 g) thick-cut bacon, diced
• 2 tbsp unsalted butter (plus extra for greasing)
• 3 cloves garlic, minced
• 2 cups (480 ml) heavy cream
• 1 cup (240 ml) whole milk
• 1 1/2 cups (150 g) shredded Gruyère cheese
• 1 cup (100 g) shredded sharp white cheddar
• 1/2 cup (50 g) grated Parmesan cheese (plus 2 tbsp extra if using topping)
• 1–2 tsp fresh thyme leaves (or 1 tsp dried)
• 1/4 tsp ground nutmeg
• 1 1/2–2 tsp kosher salt, plus more to taste
• 3/4 tsp freshly ground black pepper
• Optional topping: 1/2 cup fresh breadcrumbs + 1 tbsp melted butter

Do This

• Step 1: Preheat oven to 375°F (190°C). Butter a 9 x 13 inch (23 x 33 cm) baking dish. Peel and thinly slice potatoes; keep in cold water, then pat dry.
• Step 2: Cook diced bacon in a large skillet over medium heat until crisp. Remove to a plate; leave 1–2 tbsp drippings in the pan.
• Step 3: Add butter to the skillet, then sauté leeks and garlic with a pinch of salt until very soft and sweet, 8–10 minutes.
• Step 4: In a bowl, whisk cream, milk, thyme, nutmeg, salt, and pepper. Mix Gruyère and cheddar together in another bowl.
• Step 5: Layer half the potatoes in the dish, then half the leeks, half the bacon, and half the mixed cheeses. Repeat. Pour cream mixture evenly over the top.
• Step 6: Sprinkle Parmesan (and optional buttery breadcrumbs) over the top. Cover tightly with foil and bake 45 minutes.
• Step 7: Uncover and bake 20–25 minutes more until golden, bubbly, and potatoes are tender. Rest 15 minutes before serving.

---

Why You’ll Love This Recipe

• Deeply comforting layers of tender potatoes, sweet leeks, smoky bacon, and rich, melty cheese.
• Perfect make-ahead casserole for holidays, Sunday dinners, or cold-weather gatherings.
• Uses simple ingredients but tastes restaurant-level thanks to Gruyère, cream, and slow baking.
• Easy to customize: make it extra smoky, herby, or even partially ahead for stress-free entertaining.

Grocery List

• Produce: Yukon Gold potatoes, leeks, garlic, fresh thyme (optional but recommended)
• Dairy: Unsalted butter, heavy cream, whole milk, Gruyère cheese, sharp white cheddar, Parmesan cheese
• Pantry: Thick-cut bacon, kosher salt, black pepper, ground nutmeg, fresh breadcrumbs (or panko) if using topping

Full Ingredients

For the Gratin

• 3 lb (1.4 kg) Yukon Gold potatoes, peeled and sliced 1/8 inch (3 mm) thick
• 3 medium leeks, white and light green parts only, halved lengthwise, washed well, and thinly sliced
• 8 oz (225 g) thick-cut bacon, diced
• 2 tbsp unsalted butter, plus extra for greasing the baking dish
• 3 cloves garlic, minced
• 2 cups (480 ml) heavy cream
• 1 cup (240 ml) whole milk
• 1 1/2 cups (150 g) shredded Gruyère cheese
• 1 cup (100 g) shredded sharp white cheddar cheese
• 1/2 cup (50 g) finely grated Parmesan cheese
• 1–2 tsp fresh thyme leaves, chopped (or 1 tsp dried thyme)
• 1/4 tsp ground nutmeg
• 1 1/2–2 tsp kosher salt, to taste
• 3/4 tsp freshly ground black pepper, to taste

Optional Crisp Topping

• 1/2 cup fresh breadcrumbs (or panko)
• 1 tbsp unsalted butter, melted
• 2 tbsp finely grated Parmesan cheese
• Pinch of salt and pepper

Step-by-Step Instructions

Step 1: Preheat and Prepare the Dish

Preheat your oven to 375°F (190°C) with a rack in the center. Generously butter a 9 x 13 inch (23 x 33 cm) baking dish or similar 3-quart casserole dish to prevent sticking and to help with browning.

Peel the potatoes and slice them into 1/8 inch (3 mm) rounds. A mandoline makes this fast and even, but a sharp knife works fine. As you slice, place the potatoes in a large bowl of cold water to keep them from browning and to rinse off excess starch. When ready to layer, drain and pat the slices very dry with clean kitchen towels or paper towels so the gratin is creamy, not watery.

Step 2: Cook the Bacon

Place the diced bacon in a large, wide skillet and set it over medium heat. Cook, stirring occasionally, until the bacon is deeply browned and crisp, 8–10 minutes. Use a slotted spoon to transfer the bacon to a paper towel–lined plate, leaving about 1–2 tablespoons of bacon drippings in the pan. If there is significantly more fat than that, carefully spoon off the excess so the leeks do not become greasy.

Step 3: Sauté the Leeks and Garlic

Add the 2 tablespoons of butter to the skillet with the reserved bacon drippings. Once melted and foamy, add the sliced leeks along with a pinch of salt. Cook over medium heat, stirring frequently, until the leeks become very soft, silky, and lightly golden in spots, 8–10 minutes. You want them sweet and tender, not browned and crispy.

Stir in the minced garlic and cook for 1 minute more, just until fragrant. Turn off the heat and taste a small bit of the leek mixture; adjust the seasoning with a little more salt and pepper if needed. Set aside to cool slightly while you prepare the cream mixture.

Step 4: Mix the Cream and Cheeses

In a medium bowl or large measuring jug, whisk together the heavy cream, whole milk, thyme, nutmeg, 1 1/2 teaspoons kosher salt, and 3/4 teaspoon black pepper. This seasoned cream is what will infuse the potatoes as they bake, so make sure it tastes well-seasoned but not overly salty; remember the cheeses and bacon will add more saltiness.

In a separate bowl, combine the shredded Gruyère and shredded sharp white cheddar. Toss them together so you have a roughly even mix. Keep the grated Parmesan separate, as it will be used mainly for the top (and optional breadcrumb topping).

Step 5: Assemble the Layers

Drain the potato slices (if still in water) and pat them very dry. Arrange half of the potato slices in the bottom of the buttered baking dish, overlapping them slightly like shingles to form an even layer. Season lightly with a small pinch of salt and pepper.

Spoon half of the sautéed leeks evenly over the potatoes, then sprinkle with half of the cooked bacon. Top with half of the Gruyère–cheddar cheese mixture.

Repeat with the remaining potatoes (again overlapping slightly), another light sprinkle of salt and pepper, the rest of the leeks, the remaining bacon, and the rest of the Gruyère–cheddar mixture. Gently shake the dish to help everything settle.

Slowly pour the seasoned cream and milk mixture evenly over the entire dish, letting it seep down into the layers. Tap the dish lightly on the counter to help distribute the liquid.

Step 6: Add the Topping and Bake

Sprinkle the 1/2 cup finely grated Parmesan evenly over the surface of the gratin.

If using the optional crisp topping, in a small bowl combine the breadcrumbs, melted butter, 2 tablespoons Parmesan, and a pinch of salt and pepper. Toss until the crumbs are evenly coated, then scatter over the top of the gratin.

Cover the baking dish tightly with foil, making sure it does not touch the cheese if possible. Bake in the preheated 375°F (190°C) oven for 45 minutes. This covered phase helps the potatoes cook through and the flavors meld without over-browning the top.

Step 7: Uncover, Brown, and Rest

After 45 minutes, carefully remove the foil (watch for steam). Return the uncovered gratin to the oven and bake for an additional 20–25 minutes, or until the top is deeply golden, bubbly, and the potatoes are very tender when pierced with a knife in the center.

If the top is browning too quickly before the potatoes are tender, loosely tent it with foil again. Conversely, if you want extra color, you can place the gratin under the broiler for 1–2 minutes at the end, watching closely.

Remove the gratin from the oven and let it rest for at least 15 minutes before serving. This resting time allows the creamy sauce to thicken slightly and the layers to set, making it easier to cut neat squares. Garnish with a few extra thyme leaves if you like, then serve warm.

Pro Tips

• Slice evenly: Keeping the potatoes a consistent 1/8 inch (3 mm) thick ensures they cook at the same rate and the layers look beautiful.
• Dry the potatoes well: Excess water on the slices will thin out the cream. Pat them as dry as you reasonably can for the richest texture.
• Taste as you go: Leeks, cream, and cheese all bring different salt levels. Taste the leek mixture and cream mixture and adjust seasoning before assembling.
• Let it rest: The gratin will seem very loose straight from the oven. A 15–20 minute rest makes a huge difference in sliceability and flavor.
• Use flavorful cheese: Gruyère and sharp cheddar bring nutty, savory depth. Avoid very mild cheeses alone or the dish can taste flat.

Variations

• Extra-smoky version: Use smoked bacon (or pancetta) and add 1/4 cup smoked Gouda to the cheese mix. A small pinch of smoked paprika in the cream also works nicely.
• Herb lovers: Add 1 tablespoon finely chopped fresh herbs such as thyme, rosemary, and parsley to the cream mixture, and sprinkle more fresh herbs on top before serving.
• Meatless leek and potato gratin: Omit the bacon and use 1 extra tablespoon butter for sautéing the leeks. Add a pinch more salt to make up for the loss of bacon’s saltiness.

Storage & Make-Ahead

To store leftovers, let the gratin cool to room temperature, then cover the dish tightly or transfer portions to airtight containers. Refrigerate for up to 3 days. Reheat individual portions in the microwave, or rewarm a larger portion in a 350°F (175°C) oven, covered, for 20–25 minutes until hot and bubbling.

For make-ahead, you can fully assemble the gratin up to the point of baking, cover tightly, and refrigerate for up to 24 hours. When ready to bake, let it sit at room temperature for 20–30 minutes, then bake as directed, adding 5–10 minutes to the covered baking time if needed. You can also fully bake the gratin, cool, refrigerate, and reheat gently before serving; the texture will still be rich and creamy.

Nutrition (per serving)

Approximate values for 1 of 6 servings: about 700 calories; 45 g fat; 24 g saturated fat; 45 g carbohydrates; 4 g fiber; 8 g sugar; 23 g protein; 1,000–1,200 mg sodium. Actual values will vary based on brands used and exact portion sizes.