Preheat oven to 350°F. Brush 12 regular-size muffin cups with melted butter. Press half of a bun, cut side up, into each muffin cup.
In a large skillet, cook bacon over medium heat and transfer to paper towel-lined plate to drain. Discard drippings in pan. Add beef and onion to the skillet; heat to medium-high heat. Cook 5 to 7 minutes, stirring frequently, until beef is brown. Drain.
Divide beef mixture evenly among muffin cups and top each with 1 teaspoon ketchup. Top each one with cheese.
Bake 5-7 minutes or until cheese is melted. Immediately top with cooked bacon, followed by whatever toppings you want (relish, mustard, pickles, lettuce, tomato).