Preheat oven to 350 degrees F (175 degrees C). Lightly butter a 2 quart casserole dish.
Arrange fish fillets in the bottom of the prepared casserole dish, and sprinkle with salt and pepper to taste. Layer onion slices over fish. Spread cream of mushroom soup over all, and top with shredded cheese.
Bake in preheated oven for about 40 minutes, or until bubbly and fish flakes easily with a fork.
