HEAT grill to medium heat.
COMBINE dressing and chipotle peppers.
MIX turkey, green peppers, garlic powder and 2 Tbsp. dressing mixture just until blended; shape into 4 (1/2-inch-thick) patties.
GRILL 6 to 7 min. on each side or until done (165ºF), adding onions to the grill for the last 5 min. and grilling until charred, turning after 3 min.
PLACE burgers on bottom halves of buns; top with remaining dressing mixture and onions. Cover with tops of buns.