In a large bowl, combine the chuck, sirloin, garlic, onion, salt and pepper. Use your hands to combine the mixture and then divide it into 12 patties. Place the patties on a sheet tray, cover them with plastic wrap and refrigerate them for at least one hour (and up to 12 hours).
Preheat your grill or grill pan to medium-high heat.
Season the patties with salt and pepper.
Grill the burgers for 2 to 3 minutes on each side for medium and then add the cheese.
Brush the halved rolls with butter and place them on the grill to toast.
Place the cheeseburgers on the toasted rolls, along with the bacon, lettuce, tomato, caramelized onions and any other desired toppings.