Place filets onto the aluminum foil. Sprinkle with garlic powder, pepper, and salt. Add a few tablespoons of butter around the fish. Place onion rings on top of the filets. Sprinkle with lemon juice, if you like. Place another piece of foil about the same size as the bottom piece on top of the filets. Roll the edges to make a large "pocket". Place in the oven on about 350-375 degrees. Bake until done. Enjoy!