Preheat the oven: Preheat your oven to 375°F (190°C). Lightly grease a standard metal baking sheet or line it with parchment paper.
Brown the beef: In a large skillet over medium-high heat, add the ground beef and diced onion. Cook until the beef is fully browned and the onions are soft (about 7-8 minutes). Drain any excess grease from the pan.
Add garlic and seasonings: Stir in the minced garlic, salt, and black pepper. Cook for 1 more minute until fragrant.
Simmer the sauce: Reduce the heat to medium-low. Stir in the ketchup, brown sugar, yellow mustard, and Worcestershire sauce. Let the mixture simmer gently for 3-5 minutes until the sauce thickens and coats the meat evenly. Remove from heat.
Prep the buns: Arrange the 6 sandwich buns tightly together on the prepared baking sheet. Gently open them up without tearing the hinges.
Add the cheddar base: Take your cheddar cheese slices, cut them in half lengthwise, and press them into the bottom of each open bun. (This creates a cheese barrier so the bun doesn't get soggy).
Stuff the buns: Spoon the hot sloppy joe meat evenly into all 6 sandwich buns, directly on top of the cheddar cheese.
Top with mozzarella: Generously sprinkle the shredded mozzarella cheese over the top of the meat on all the sandwiches.
Bake: Place the baking sheet in the oven and bake for 8-10 minutes, or until the mozzarella cheese is fully melted, bubbly, and slightly browned on top.
Serve: Remove from the oven and let them cool for 2 minutes before serving. Enjoy hot!