COOK onions in hot oil in large nonstick skillet on medium-high heat 8 min. or until golden brown, stirring frequently and adding barbecue sauce the last few minutes. Remove onions from skillet; set aside. Wipe skillet with paper towel.
SHAPE meat into 4 (1/2-inch-thick) patties. Cook in skillet on medium heat 4 to 6 min. on each side or until done (160ºF). Top with Singles; cover. Remove from heat. Let stand until melted.
FILL toast slices with burgers and onions.
