Combine ground round, 1/4 cup barbecue sauce, breadcrumbs, onion flakes, and pepper; shape mixture into 4 patties.
Coat rack of a broiler pan with cooking spray. Place patties and onion slices on rack; broil 5 1/2 inches from heat (with electric oven door partially opened) 8 minutes, brushing occasionally with remaining 1/4 cup barbecue sauce.
Place a patty and an onion slice on bottom half of each bun. Top with remaining bun halves.
Cooking Light Low-Fat, Low-Calorie Quick and Easy Cookbook