Heat gas or charcoal grill. In medium bowl, mix ground beef, salt and pepper. Shape mixture into 4 oval patties, about 1/2 inch thick, to fit hoagie buns.
Cut 2 (18x12-inch) sheets of heavy-duty foil. Generously spray half of one side of each foil sheet with cooking spray. Place onions evenly in center of sprayed portion of foil; generously spray onions with cooking spray. Fold unsprayed half of foil over onions so edges meet; seal edges with tight 1/2-inch folds and fold again, allowing room for heat expansion.
When grill is heated, place patties and packets on gas grill over medium heat or on charcoal grill over medium coals; cover grill. Cook 11 to 13 minutes, turning patties and packets over once, until meat thermometer inserted in center of patties reads 160°F and onions are tender.
Spread cheese onto bottom halves of buns. Top each with lettuce leaf, patty and onions. Cover with top halves of buns.