Heat gas or charcoal grill. In medium bowl, mix beef, mustard and garlic. Shape mixture into 4 patties, about 3/4 inch thick.
Place patties on grill over medium heat. Cover grill; cook 13 to 15 minutes, turning once, until meat thermometer inserted in center of patties reads 160°F. Add buns, cut sides down, to grill for last 4 minutes of grilling or until toasted. Top burgers with cheese.
Serve burgers on buns with lettuce and red peppers.