- Preheat oven to 350° and grease a muffin pan with cooking spray. Roll out burger bun halves to 1/4” thick. Place the rolled-out buns cut side up in muffin cups.
- In a large skillet over medium-high heat, heat oil. Add onion and cook until tender, about 5 minutes. Add ground beef, breaking the meat up with a wooden spoon, and cook until the beef is no longer pink, about 6 minutes. Drain.
- Season mixture with garlic powder, salt and pepper. Cut each cheddar slice into 4 small squares. Divide ground beef mixture between burger bun cups. Top with a slice of cheddar and bake until the cheese has melted and the edges of the bun are golden, about 10 minutes.
- Drizzle ketchup and mustard on top and garnish with a pickle, if desired