Melt 2 Tbsp. butter in a medium skillet over medium heat. Add onion and 1/4 tsp. salt. Cook, stirring often, 15 to 20 minutes or until onions are caramel colored.
Arrange desired bread slices on an aluminum foil-lined 15- x 10-inch jelly-roll pan; layer with turkey, roast beef, and ham. Top with Parmesan Cheese Sauce and onions; sprinkle with Parmesan cheese.
Broil 6 inches from heat 4 to 6 minutes or until bubbly and lightly browned; remove from oven. Top with crumbled bacon and tomato slices; serve immediately.