Potatoes and wild mushrooms layered and baked with cheese create an earthy Italian gratin. A vegetarian mediterranean dish that's hearty, family-friendly, and perfect for healthy comfort food.
Preheat your oven to 375°F and grease a 9x13 inch baking dish with 1 tbsp olive oil.
Slice the russet potatoes thinly (about 1/8 inch) and soak them in cold water for 10 minutes to remove excess starch.
Heat 2 tbsp olive oil in a large skillet over medium-high; sauté finely chopped onion until translucent, then add sliced wild mushrooms and fresh thyme, cooking until the mushrooms are browned and moisture evaporates (about 7 minutes).
Drain potatoes and pat them dry; layer half of the potatoes in the baking dish, sprinkle with salt, pepper, and half the nutmeg, then spread half of the mushroom mixture and half the Fontina cheese over them.
Repeat layering with remaining potatoes, seasoning, mushrooms, and cheese. Pour whole milk evenly over the layers, cover the dish tightly with foil, and bake for 40 minutes. Uncover and continue baking for 15–20 minutes until golden brown and bubbling. Let rest 10 minutes before serving.