Creamy Mushroom and Potato Gratin

Preheat oven to 375 degrees F (190 degrees C) and butter a large 3-quart casserole dish.

Melt butter in a large skillet over medium-high heat until starting to foam.

Add mushrooms and cook until lightly browned and the moisture has evaporated.

Add garlic and continue to cook, stirring often for 1-2 minutes. Set aside.

Whisk together heavy cream, sour cream, kosher salt, ground black pepper, and ground nutmeg.

In a separate bowl, whisk together milk and flour until smooth. Add to the sour cream and heavy cream mixture and whisk until incorporated.

Spread half of the potato slices evenly on the bottom of the prepared casserole dish.

Scatter half of the sautéed mushrooms and half of the Gruyère cheese evenly over the potatoes.

Repeat with remaining potatoes, mushrooms, and cheese.

Pour the cream mixture evenly over the potatoes. Give the pan a gentle shake to help flatten everything out.

Cover and bake for 45-60 minutes, checking at 45 minutes. The potatoes should be almost tender in the center.

Remove the cover and bake for an additional 20 minutes until the top is golden.

Remove from oven and allow to rest for 10 minutes before serving.

Garnish with fresh parsley if desired.