HEAT oil in large nonstick skillet on medium-high heat. Add onions; cook 7 to 9 min. or until tender and golden brown, stirring frequently and adding barbecue sauce the last few minutes. Remove onions from skillet; cover to keep warm. Carefully wipe skillet with paper towel.

SHAPE meat into 4 (½-inch-thick) patties. Add to skillet; cook on medium heat 4 to 6 min. on each side or until done (160°F). (I cooked mine in the grill and seasoned the patties with garlic salt and pepper) Top with Singles; cover. Remove from heat. Let stand until melted. Meanwhile, toast bread.

FILL toast slices with remaining ingredients.