HEAT grill to medium-high heat.
MIX meat, onions and dressing; shape into 4 (1/2-inch-thick) patties.
GRILL 4 min. on each side or until done (160°F). Top with 2% Milk Singles. Place buns, cut-sides down, on grate of grill next to burgers. Grill 1 min. or until buns are lightly toasted and Singles are melted.
MIX ketchup and seasoning. Place burgers on bottom halves of buns; cover with ketchup mixture and tops of buns.