1Heat coals or gas grill for direct heat. Spray two 18x12-inch sheets of heavy-duty foil with cooking spray.
2Brush beef patties with steak sauce; sprinkle with half of the soup mix (dry). Place half of the onions on center of each foil piece. Sprinkle with remaining soup mix, the brown sugar and vinegar. Fold foil over onions so edges meet. Seal edges, making tight 1/2-inch fold; fold again. Allow space on sides for circulation and expansion.
3Cover and grill packets and beef patties 4 to 6 inches from medium heat 10 to 15 minutes, turning patties and rotating packets 1/2 turn once or twice, until meat thermometer inserted in center of patties reads 160ºF. Place packets and patties on plates. Cut large X across top of each packet; fold back foil.