Melt butter in large skillet on medium heat. Add onions; cover. Cook 9 min. or until onions are tender, uncovering and stirring frequently after 3 min. Stir in garlic; cook 8 to 10 min. or until onions are golden brown, stirring occasionally. Stir in 3 Tbsp. barbecue sauce and sugar; cook and stir 2 min.