When bread is done baking, set oven control to broil. Cut each loaf in half horizontally, cutting to but not completely through one long side; place cut sides up on cookie sheet. Spread 1/2 cup mayonnaise mixture over cut sides of each loaf. Using slotted spoon, remove beef and onion from broth mixture, reserving broth mixture. Top each loaf with half of the beef and onion; top with cheese.