Preheat oven to 375 degrees F (190 degrees C) and butter a large 3-quart casserole dish.
Salted Butter (2 Tbsp)
in a large skillet over medium-high heat until starting to foam. Add
Cremini Mushrooms (4 1/2 cups)
and cook until lightly browned and the moisture has evaporated.
Garlic (3 cloves)
and continue to cook, stirring often for 1-2 minutes. Set aside.
While mushrooms are cooking, whisk together the
Heavy Cream (1 cup)
Sour Cream (1 cup)
Kosher Salt (1 tsp)
Ground Black Pepper (1/2 tsp)
Ground Nutmeg (1 pinch)
In a separate bowl whisk together the
Milk (1/3 cup)
All-Purpose Flour (2 Tbsp)
until smooth. Add to the sour cream and heavy cream and whisk until incorporated.
Peel and thinly slice
Russet Potatoes (7 1/2 cups)
Spread half of the potato slices evenly on the bottom of the prepared casserole dish. Scatter half of the sauteed mushroom and half of the
Gruyère Cheese (1 1/2 cups)
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 (this will depend on how thin you've sliced the potatoes). Start 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 (to taste)