Prepare outdoor grill for direct grilling over medium-high heat. In large bowl, gently mix beef, salt and pepper; do not overmix. Form beef mixture into 4 (1/2-inch-thick) patties; spray with cooking spray.
Place patties on hot grill rack; cover and cook 10 minutes or until internal temperature reaches 160°, turning once. About 2 minutes before burgers are done, top burgers with cheese; transfer burgers to cutting board and loosely cover with aluminum foil.
Spray both sides of tomatoes and onions, and cut sides of buns with cooking spray; place onions on hot grill rack, cover and cook 2 minutes. Place tomatoes and buns, cut side down, on hot grill rack and turn onions; cover and cook 3 minutes or until grill marks appear on vegetables and buns are lightly toasted.
Serve burgers in buns topped with onions, lettuce, tomatoes and dressing.