Mushroom and Potato Gratin

Ingredients

• Potatoes (4 large, peeled and thinly sliced)
• Mushrooms (2 cups, sliced (preferably cremini or button mushrooms))
• Onion (1 medium, chopped)
• Garlic (2 cloves, minced)
• Unsalted butter (4 tablespoons, divided)
• All-purpose flour (2 tablespoons)
• Milk (2 cups)
• Gruyere cheese (1 cup, grated)
• Parmesan cheese (1/2 cup, grated)
• Fresh thyme (1 teaspoon, chopped (or 1/2 teaspoon dried thyme))
• Salt (to taste)
• Black pepper (to taste)

Instructions

1. Preheat the oven to 375°F (190°C).
2. In a large skillet, melt 2 tablespoons of butter over medium heat. Add the sliced mushrooms, chopped onion, and minced garlic. Sauté until the mushrooms are golden brown and the onion is soft, about 8-10 minutes.
3. In a separate saucepan, melt the remaining 2 tablespoons of butter over medium heat. Whisk in the flour and cook for 1-2 minutes, until lightly golden.
4. Gradually whisk in the milk, continuing to cook and whisk until the mixture thickens, about 5 minutes.
5. Stir in the grated Gruyere cheese and half of the grated Parmesan cheese until melted and smooth. Season the sauce with salt, pepper, and fresh thyme to taste.
6. In a greased baking dish, layer half of the sliced potatoes evenly at the bottom. Spread half of the mushroom mixture over the potatoes, then pour half of the cheese sauce over the mushrooms.
7. Repeat the layering process with the remaining potatoes, mushrooms, and cheese sauce. Top with the remaining Parmesan cheese.
8. Cover the baking dish with foil and bake for 30 minutes.
9. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.
10. Allow the gratin to cool for a few minutes before serving. Enjoy!