Heat grill to medium heat.
Cook bacon in small skillet until crisp. Meanwhile, cut 4 thin slices from center of onion; finely chop remaining onion.
Remove bacon from skillet, reserving 1 Tbsp. drippings in skillet. Drain bacon on paper towels. Add chopped onions to reserved drippings; cook on medium heat 5 to 6 min. or until golden brown, stirring frequently. Remove from heat.
Crumble bacon; mix with cooked onions and cheese. Shape meat into 8 thin patties. Top 4 patties with bacon mixture; cover with remaining patties. Pinch edges together to seal.
Grill 6 to 7 min. on each side or until done (160ºF); top with Singles. Fill buns with burgers and sliced onions.