Indulge in the ultimate comfort food: a creamy, cheesy potato and mushroom gratin. This hearty dish is perfect for a cozy night in or a special occasion.
INGREDIENTS
- 3-4 tbsp unsalted butter (plus extra for greasing the dish)
- 16 oz mushrooms, thinly sliced (see note)
- 1 small onion, thinly sliced
- 3 cloves garlic, minced
- 1 cup heavy cream
- 1 cup full-fat sour cream
- 1 tsp kosher salt
- 1/2 tsp pepper
- Pinch of nutmeg
- 1/3 cup milk
- 2 tbsp flour
- 2 1/2 lbs russet potatoes (see note)
- 6 oz Gruyère cheese, shredded
INSTRUCTIONS
- Preheat the oven to 375°F and butter a large 3-quart casserole dish.
- In a large skillet, melt 2 tbsp of butter over medium-high heat until it begins to foam. Add the mushrooms and cook until lightly browned and the moisture has evaporated. Add the minced garlic and cook, stirring often, for 1-2 minutes. Season with salt and pepper, then transfer to a clean bowl.
- In the same skillet, melt another 1-2 tbsp of butter over medium heat. Add the sliced onions, season with a pinch of salt, and sauté for 4-5 minutes, until they begin to soften.
- While the mushrooms and onions are cooking, whisk together the heavy cream, sour cream, salt, pepper, and nutmeg. In a separate bowl, whisk together the milk and flour until smooth. Add this mixture to the sour cream blend and whisk until well combined. Set aside the grated Gruyère cheese.
- Prepare the potatoes: Peel and thinly slice the potatoes, then pat them dry with a paper towel. Layer half of the potato slices in the bottom of the prepared casserole dish. Top with half of the sautéed mushrooms and onions, followed by half of the shredded cheese. Repeat with the remaining potatoes, mushrooms, and cheese.
- Pour the cream mixture evenly over the layered ingredients. Gently shake the pan to even everything out. Cover the dish and bake for 45-60 minutes, depending on the thickness of the potato slices (check at 45 minutes). The potatoes should be just tender when pierced with a fork.
- After 45-60 minutes, uncover the dish and bake for an additional 20 minutes, or until the top is golden and the potatoes are fully cooked.
- Remove from the oven and cover with foil. Let it rest for 10 minutes before serving. If desired, garnish with freshly chopped parsley or sliced green onions.
NOTES
- The mushrooms, onions, and cream sauce can be prepared ahead of time, up to a day in advance. Then, you just need to slice the potatoes, assemble, and bake.
- Russet potatoes are ideal for this recipe, though red potatoes can be used as well with good results.
- The covered cooking time may vary depending on how thin you slice the potatoes. Hand-slicing typically requires closer to 60 minutes of covered baking.
- A combination of cremini and white mushrooms works wonderfully, but you can use either variety on its own.
