Combine first 9 ingredients in a bowl; stir well. Divide mixture into 4 equal portions, shaping into 1/2-inch-thick patties.
Prepare grill. Place patties on grill rack coated with cooking spray; grill 6 minutes on each side or until done. Place 1 cheese slice on top of each patty; cover and grill an additional minute or until cheese melts.
Spread 1 tablespoon of sour cream over top half of each bun, and set aside. Place patties on bottom halves of buns; top each with lettuce, tomato, onions (if desired), and top half of bun.