In a small skillet, heat oil over medium-high heat. Add sliced onion; cook and stir 4-6 minutes or until tender., In a large bowl, combine beef, chopped onion, barbecue sauce, garlic, onion powder, salt and pepper, mixing lightly but thoroughly. Shape into four 1/2-in.-thick patties., Grill burgers, covered, over medium heat or broil 4 in. from heat 4-6 minutes on each side or until a thermometer reads 160°. Serve on buns with cooked onions and, if desired, optional toppings.