Spread mayonnaise on one 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 large skillet, fry eggs over medium heat until completely set; place over bacon. Top with tomato slices; set aside.
In a small saucepan, melt butter. Stir in the flour, salt and pepper until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened.
Pour over tomatoes. Sprinkle with cheddar cheese and onions. Bake, uncovered, at 325° for 10 minutes. Cut in squares; serve with shredded lettuce.