01 -
In a large skillet over medium heat, cook the ground beef with the chopped onion, breaking up the meat as it browns. Drain excess fat and transfer the mixture to a slow cooker.
02 -
Add garlic powder, salt, black pepper, yellow mustard, ketchup, and Worcestershire sauce into the slow cooker. Stir to combine the ingredients evenly. Place the cubed American cheese and shredded cheddar cheese on top of the mixture.
03 -
Cover the slow cooker and cook on low for 2 to 3 hours, stirring occasionally, until the cheeses are fully melted and the mixture is smooth and creamy.
04 -
Serve the hot cheeseburger mixture on toasted hamburger buns while warm.