Potato-Mushroom Pie

Ingredients

• 2 tablespoons unsalted butter, plus more for dish
• 1 medium onion, cut into 1/2-inch dice
• 2 pounds assorted wild mushrooms, such as chanterelle, oyster, or black trumpet, coarsely chopped
• 1/4 cup dry white wine, such as sauvignon blanc
• 1 1/2 teaspoons coarse salt
• freshly ground pepper
• 1 cup finely grated gruyere cheese (about 4 ounces)
• 1 cup finely grated parmesan cheese, about 3 1/2 ounces
• 2 pounds yukon gold potatoes, peeled and cut into 1/8-inch-thick rounds
• 1 tablespoon finely chopped thyme leaves
• 3/4 cup whole milk
• 1/2 cup heavy cream