Creamy Thyme Potato and Mushroom Bake

Quick Recipe Version (TL;DR)

• Yield: 6 servings
• Prep Time: 25 minutes
• Cook Time: 55 minutes
• Total Time: 1 hour 20 minutes

Quick Ingredients

• 2 lb (900 g) Yukon Gold or Russet potatoes, thinly sliced
• 1 lb (450 g) cremini or button mushrooms, sliced
• 1 large yellow onion, thinly sliced
• 6 tbsp unsalted butter, divided
• 2 tbsp olive oil
• 3 cloves garlic, minced
• 3 cups (720 ml) whole milk + heavy cream (2 cups milk, 1 cup cream)
• 3 tbsp all-purpose flour
• 2½–3 tsp kosher salt, divided
• 1 tsp freshly ground black pepper, divided
• 2–3 tsp fresh thyme leaves (or 1–1½ tsp dried), divided
• 1 tsp Dijon mustard
• Pinch ground nutmeg (optional)
• 2½ cups (about 250 g) shredded cheese (Gruyère, cheddar, and Parmesan mix)
• Chopped fresh parsley for garnish (optional)

Do This

• 1. Preheat oven to 375°F (190°C). Butter a 9×13-inch (3 L) baking dish. Thinly slice potatoes, mushrooms, and onion.
• 2. Sauté onion and mushrooms in 3 tbsp butter + 2 tbsp oil until golden and most liquid evaporates; add garlic, thyme, salt, and pepper.
• 3. Make sauce: in a saucepan melt 3 tbsp butter, whisk in flour 1 minute, then slowly whisk in milk and cream. Add salt, pepper, thyme, Dijon, and nutmeg; simmer until thick.
• 4. Layer half the potatoes in the dish, season lightly with salt and pepper. Top with half the mushroom-onion mixture and about one-third of the cheese. Repeat layers.
• 5. Pour hot thyme cream sauce evenly over everything, then sprinkle the remaining cheese on top. Cover tightly with foil.
• 6. Bake covered 35 minutes, then uncover and bake 20 minutes more, until potatoes are tender and top is bubbly and browned. Rest 10 minutes before serving.

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Why You’ll Love This Recipe

• All the cozy comfort of scalloped potatoes with extra depth from golden sautéed mushrooms and onions.
• A creamy thyme sauce and blanket of melted cheese make every bite rich, savory, and deeply satisfying.
• Simple, familiar ingredients but impressive enough for guests or a holiday table.
• Easy to prep ahead, reheats beautifully, and works as a hearty side or a vegetarian main.

Grocery List

• Produce: Yukon Gold or Russet potatoes, cremini or button mushrooms, yellow onion, garlic, fresh thyme (or dried thyme), fresh parsley (optional).
• Dairy: Unsalted butter, whole milk, heavy cream (or half-and-half), Gruyère or Swiss cheese, sharp cheddar or Fontina, Parmesan cheese.
• Pantry: Olive oil, all-purpose flour, Dijon mustard, kosher salt, black pepper, ground nutmeg (optional).

Full Ingredients

For the Potato and Mushroom Layers

• 2 lb (900 g) Yukon Gold or Russet potatoes, peeled if desired and sliced 1/8 inch (3 mm) thick
• 1 lb (450 g) cremini or button mushrooms, cleaned and sliced
• 1 large yellow onion, thinly sliced
• 3 tbsp unsalted butter
• 2 tbsp olive oil
• 3 cloves garlic, minced
• 1–1½ tsp fresh thyme leaves, finely chopped (or 1/2 tsp dried thyme)
• 1 tsp kosher salt (for seasoning the mushroom mixture)
• 1/2 tsp freshly ground black pepper (for seasoning the mushroom mixture)
• Additional kosher salt and pepper for seasoning the potato layers (about 1–1½ tsp salt and 1/2 tsp pepper total)

For the Creamy Thyme Sauce

• 3 tbsp unsalted butter
• 3 tbsp all-purpose flour
• 2 cups (480 ml) whole milk, at room temperature if possible
• 1 cup (240 ml) heavy cream (or half-and-half)
• 1–1½ tsp fresh thyme leaves, finely chopped (or 1/2 tsp dried thyme)
• 1½ tsp kosher salt (adjust to taste)
• 1/2 tsp freshly ground black pepper
• 1 tsp Dijon mustard
• Pinch ground nutmeg (optional, but lovely with the cream and potatoes)

For the Cheesy Topping

• 1½ cups (150 g) shredded Gruyère or Swiss cheese
• 1 cup (100 g) shredded sharp cheddar or Fontina cheese
• 1/4 cup (25 g) finely grated Parmesan cheese
• 1–2 tbsp chopped fresh parsley, for garnish (optional)

Step-by-Step Instructions

Step 1: Preheat the oven and prepare the dish

Preheat your oven to 375°F (190°C) with a rack in the center. Lightly butter a 9×13-inch (about 3-quart / 3 L) baking dish or similar casserole. This helps prevent sticking and makes cleanup easier.

Peel the potatoes if you prefer (Yukon Gold skins are thin and can be left on if well-scrubbed). Using a sharp knife or mandoline, slice the potatoes into 1/8-inch (3 mm) rounds. Try to keep the slices as even as possible so they cook at the same rate. Set aside; do not soak them, or they will lose some of their starch, which helps thicken the sauce.

Step 2: Sauté the onions and mushrooms until golden

In a large, wide skillet, heat 3 tbsp unsalted butter and 2 tbsp olive oil over medium-high heat. Add the sliced onions and cook for 3–4 minutes, stirring often, until they begin to soften and turn translucent.

Add the sliced mushrooms in an even layer. At first, the pan will look crowded and the mushrooms will release a lot of moisture. Continue to cook, stirring every couple of minutes, for 8–10 minutes, until most of the liquid has evaporated and the mushrooms are deeply browned around the edges.

Stir in the minced garlic, 1–1½ tsp fresh thyme (or 1/2 tsp dried), 1 tsp kosher salt, and 1/2 tsp black pepper. Cook for 1 minute more, just until fragrant. Remove the pan from the heat and set the mixture aside.

Step 3: Make the creamy thyme sauce

In a medium saucepan, melt 3 tbsp unsalted butter over medium heat. Sprinkle in the 3 tbsp flour and whisk constantly for about 1 minute. The mixture (a roux) should bubble gently and smell slightly nutty, but not brown. This step cooks out the raw flour taste.

Gradually pour in the 2 cups milk and 1 cup cream, whisking constantly to avoid lumps. Continue whisking until the mixture is smooth. Add 1–1½ tsp fresh thyme (or 1/2 tsp dried), 1½ tsp kosher salt, 1/2 tsp black pepper, 1 tsp Dijon mustard, and a pinch of nutmeg if using.

Bring the sauce to a gentle simmer, stirring often, for 4–6 minutes, until it thickens to the consistency of a light gravy and coats the back of a spoon. Taste and adjust seasoning if needed; it should be well seasoned now, since it will flavor all the potatoes. Remove from the heat.

Step 4: Prepare the cheese blend

In a medium bowl, combine 1½ cups shredded Gruyère (or Swiss), 1 cup shredded sharp cheddar (or Fontina), and 1/4 cup grated Parmesan. Toss gently to mix. Set aside. Having the cheeses pre-mixed makes layering much easier and helps them melt evenly.

Step 5: Build the first layers in the baking dish

Arrange half of the sliced potatoes in the prepared baking dish in slightly overlapping rows, like shingles on a roof. Aim for an even layer so the potatoes cook uniformly. Lightly season this layer with a pinch or two of salt and pepper (about 1/2 tsp salt and 1/4 tsp pepper total).

Spoon half of the mushroom and onion mixture evenly over the potatoes, spreading it all the way to the corners so each serving gets some. Sprinkle about one-third of the cheese mixture over the mushroom layer.

Step 6: Finish layering and add the sauce and cheese

Repeat with the remaining potatoes, arranging them in overlapping rows on top of the cheese. Lightly season again with a pinch or two of salt and pepper. Top with the remaining mushroom and onion mixture and another one-third of the cheese.

Give the warm cream sauce a quick stir, then slowly pour it evenly over the entire dish, letting it trickle down through the layers. Gently tap the dish on the counter to help the sauce settle between the potatoes.

Scatter the remaining cheese mixture evenly over the top. Cover the baking dish tightly with aluminum foil, tenting it slightly so it does not stick to the cheese.

Step 7: Bake until tender, then brown and rest

Place the covered dish on the center rack of the preheated 375°F (190°C) oven. Bake for 35 minutes, covered, to allow the potatoes to soften and the sauce to bubble.

After 35 minutes, carefully remove the foil and continue baking, uncovered, for 18–22 minutes more, until the top is deeply golden and bubbly and a sharp knife slides easily through the potatoes with no resistance in the center.

Remove the bake from the oven and let it rest for at least 10 minutes before serving. This rest time allows the sauce to thicken slightly and the layers to set, so the dish slices more neatly. Garnish with chopped fresh parsley if desired, then serve warm, scooping down through all the layers.

Pro Tips

• Slice potatoes evenly: A mandoline set to 1/8 inch (3 mm) thickness makes quick work of slicing and ensures even cooking. If slicing by hand, take your time and aim for consistent slices.
• Cook off mushroom moisture: Do not rush the mushroom sauté. Let most of the liquid evaporate so you get deep, savory flavor instead of a watery bake.
• Season each layer: Lightly salting the potato layers helps keep the dish well balanced. If you only season the sauce, the potatoes can taste flat.
• Use freshly shredded cheese: Pre-shredded cheese often contains anti-caking agents that prevent smooth melting. Grating the cheese yourself makes the top beautifully gooey and golden.
• Check doneness in the center: The middle is the last part to cook. Slide a thin knife all the way down; if it meets resistance, bake for another 5–10 minutes and check again.

Variations

• Smoky bacon version: Cook 4–6 slices of chopped thick-cut bacon until crisp, then sauté the onions and mushrooms in some of the bacon fat. Sprinkle the cooked bacon between the layers with the mushrooms.
• Herb and leek twist: Replace the onion with 2 thinly sliced leeks (white and light green parts only) and add a teaspoon of chopped fresh rosemary along with the thyme.
• Lighter option: Use half-and-half instead of heavy cream, and reduce cheese to 1½ cups total. The dish will be a bit less rich but still very comforting.

Storage & Make-Ahead

To make ahead, assemble the entire dish through Step 6, cover tightly with foil, and refrigerate for up to 24 hours before baking. When ready to bake, remove from the fridge while the oven preheats; you may need to add 5–10 extra minutes to the covered baking time if it goes into the oven cold.

Leftovers keep well. Let the bake cool completely, then cover the dish or transfer portions to airtight containers and refrigerate for up to 4 days. Reheat individual portions in the microwave or rewarm the whole dish, covered with foil, in a 325°F (165°C) oven for 20–25 minutes, until hot throughout.

For longer storage, you can freeze tightly wrapped portions for up to 2 months. Thaw overnight in the refrigerator, then reheat in the oven for best texture.

Nutrition (per serving)

Approximate values for 1 of 6 servings: about 520 calories; 33 g fat; 19 g saturated fat; 39 g carbohydrates; 4 g fiber; 8 g sugar; 15 g protein; 930 mg sodium. Actual values will vary based on specific ingredients and brands used.