MIX 1 cup Kraft Garlic BarBQ Sauce and 3 Tbsp. balsamic vinegar until well blended; set aside.

PIERCE both sides of one 1 lb. beef flank steak (1-inch-thick) evenly with fork; place in shallow glass dish. Pour barbecue sauce mixture over steak; cover. Refrigerate 12 to 24 hours to marinate. Drain; discard marinade.

PREHEAT barbecue to medium-high heat. Grill steak 5 min. on each side or until desired doneness (medium-rare 140°F or medium 160°F). Cut into thin slices to serve.