HEAT grill to medium heat.
CHOP 2 chiles; mix with meat and garlic. Shape into 6 (1/2-inch-thick) patties.
GRILL patties and onions 4 to 5 min. on each side or until patties are done (160ºF). Meanwhile, cut remaining chiles into strips. Top burgers with Singles; grill until Singles are melted.
SPREAD buns with mayo; fill with cheeseburgers, chili strips and onions, separated into rings.