HEAT grill to medium heat.
MIX first 4 ingredients just until blended; shape into 18 (1-inch) balls. Flatten slightly to form patties.
SPRAY large sheet of foil with cooking spray; place on grill grate. Add patties; grill 2 to 3 min. on each side or until done (160ºF).
THREAD 1 each burger, cheese slice and tomato onto each of 18 toothpicks.