How to Make It
Cook bacon in a large nonstick skillet over medium, stirring occasionally, until browned and crisp. Transfer to a plate; set aside. Pour off half of the bacon drippings from skillet, and reserve.
Add onions to skillet with remaining drippings over medium-low; cook, stirring occasionally, until browned and caramelized, about 30 minutes. Stir in Worcestershire, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper. Transfer to a bowl; set aside.
Preheat oven to 350°F. Heat skillet over medium-high. Add ground beef, 1/2 teaspoon of the pepper, and remaining 3/4 teaspoon salt; cook, stirring occasionally, until browned, 8 to 10 mintues. Stir in ketchup and mustard; remove from heat.
Spread beef mixture in bottom of a lightly greased 3-quart baking dish. Top with caramelized onions and crumbled bacon. Arrange cheese slices over bacon.
Toss bun cubes in reserved bacon drippings, and place in an even layer over cheese slices. Sprinkle with cheddar cheese and remaining 1/4 teaspoon pepper. Bake in preheated oven until golden and bubbly, about 20 minutes. Serve with desired toppings.