HEAT oven to 350ºF. Mix meat, onions, ketchup and mustard just until blended.

ARRANGE bottom halves of buns, cut-sides up, in 13x9-inch baking pan sprayed with cooking spray. Spoon about 1 Tbsp. of the meat mixture onto each; flatten with spatula. Top with pickles, Singles and tops of buns. Cover tightly with foil.

BAKE 30 to 35 min. or until meat mixture is cooked through. Let stand 5 min. before serving.