Heat grill to medium-high heat.
Shape meat into 8 (1/4-inch thick) patties. Top each of 4 patties with 4 Singles pieces; cover with remaining patties. Pinch edges together to seal.
Grill burgers and onions 6 to 7 min. on each side or until burgers are done (160ºF) and onions are tender.
Fill buns with burgers, onions and remaining ingredients.