In a large skillet, saute onion in butter until tender. Whisk the eggs, tomato, salt and pepper sauce. Add egg mixture to skillet (mixture should set immediately at edges). As eggs set, push cooked edges toward the center, letting uncooked portion flow underneath. Cook until eggs are set. Meanwhile, heat bacon in the microwave and toast bagels if desired. Layer bagel bottoms with cheese. Cut omelet into fourths and serve on bagels with bacon.