Begin by cooking ground beef and diced onion in a skillet until the beef is browned and cooked through. Drain any excess fat.
Stir in cooked and crumbled bacon, chopped pickles, ketchup, mustard, and Worcestershire sauce to the beef mixture.
Butter one side of the bread slices and place them buttered side down in a casserole dish to create a base layer.
Spread the beef mixture evenly over the bread layer in the casserole dish.
Place slices of cheddar cheese over the beef mixture, followed by a layer of shredded mozzarella cheese.
Place another layer of buttered bread slices on top of the cheese layer, buttered side facing up
Bake the Bacon Cheeseburger Grilled Cheese Casserole in the oven until the top is golden brown and the cheese is melted and bubbly.