Preheat oven to 325°. Spread mayonnaise on 1 side of each slice of toast and cut into small pieces. Arrange toast, mayonnaise side up, in a greased 8-in. square baking dish. Top with cheese slices and bacon., In a small saucepan, melt butter. Stir in flour, salt and pepper until smooth. Gradually add milk. Bring to a boil; cook and stir until thickened, 2 minutes. Pour over bacon., In a large skillet, fry eggs over medium heat until they reach desired doneness; place over bacon. Top with tomato slices; sprinkle with cheddar cheese and onions. Bake, uncovered, 10 minutes. Cut in squares; serve with lettuce.