In a small skillet, add 1 tablespoon butter or oil over medium heat. Add onion and cook until softened, about 10 minutes. Transfer to a large bowl and cool slightly.

To the bowl, add the cheese, mayonnaise, hot sauce, and half of each of the salt, pepper and garlic powder.  Mix well and set aside. Grease a 13 by 9 by 2-inch baking dish. Sprinkle the fish with the remaining salt, pepper and garlic powder.  Place the fish in the baking dish and squeeze the juice of the lemon over it and sprinkle with red pepper flakes. Top fish with slices of butter. Bake for about 10 to 15 minutes or until fish is almost done; the time will depend on the thickness of your fish. Remove the dish from the oven and cover each fillet with 1/2 cup of the cheese mixture. Return to the oven and bake until cheese melts, about 8 to 10 minutes.