Heat gas or charcoal grill. In large bowl, mix beef and onion mixture. Shape mixture into 4 patties, 1/4 inch thick. Place patties on grill over medium heat. Cover grill; cook 5 to 8 minutes, turning once, until meat thermometer inserted in center of patties reads 160°F. During last 2 minutes of cooking, place rolls, cut sides down, on grill until toasted.