HEAT broiler.
MIX meat, barbecue sauce and onions; shape into 8 (1/2-inch-thick) patties.
BROIL 3 min. on each side or until done (160ºF).
CUT pitas horizontally in half to open pockets, being careful to not cut through to opposite side of each. Spread insides of pockets with dressing; fill with tomatoes, burgers and Singles.