Start by heating a large skillet over medium-high heat. Add the ground beef and break it up with a spatula as it cooks. Once it starts to brown, toss in the diced onions and continue cooking until the onions are soft and the beef is fully browned. Drain any excess grease to keep the filling from becoming too oily.

Once drained, return the beef to the pan and add ketchup, mustard, Worcestershire sauce, salt, black pepper, and garlic powder. Stir everything together and let it cook for another minute or two to ensure all the flavors meld. Remove from heat and let the mixture cool slightly before adding the cheese. Stir in the shredded cheddar and mozzarella until they melt into the beef mixture, creating a rich, cheesy filling.