Season potatoes with half the Parmesan, ¼ tsp. salt, and a pinch of pepper. Top with mushrooms and cover with remaining potatoes. Season again with remaining Parmesan, ¼ tsp. salt, and a pinch of pepper.
Return pan to medium heat until butter is bubbling around edges, 1 minute. Cover, and place pan in hot oven. Bake until tender, 15-18 minutes.
Uncover, and bake until top lightly browns, 5 minutes.
Carefully remove from oven and rest 2 minutes.
While potatoes bake, cook meatloaves.