Preheat your oven to 425ºF.  Coat a 13 x 9 x2 inch baking pan, lightly with oil. Layer in bottom of pan half of the onion slices. Place the fish on top of the onions. Sprinkle with pepper and garlic powder. Cover with chopped tomato and remaining onion slices. Dot the top of onions with Tabasco sauce. Bake at 425ºF. for 20 to 25 minutes or until the fish flakes easily with a fork.