Preheat oven to 350 degrees F (175 degrees C). Rub butter all over a 2-quart baking dish.
Heat a large skillet over medium heat. Add ground beef, pepper, onion powder, and salt; cook and stir until browned, 5 to 8 minutes.
Mix cream of mushroom soup and sour cream together in a bowl.
Lay 6 saltine crackers in the bottom of the prepared baking dish. Top with 1/2 the ground beef. Dollop 1/3 of the mushroom soup mixture over beef. Lay remaining saltines on top. Cover with spinach and another 1/3 of the mushroom soup mixture. Spread remaining ground beef and mushroom soup mixture on top. Cover with mushrooms.
Bake in the preheated oven until bubbly, about 30 minutes. Top with Muenster cheese and continue baking until cheese is melted, about 5 minutes more.