MIX 1/3 cup of the steak sauce, the onions, rosemary, lemon zest, garlic and thyme. Place steak in nonmetal dish or resealable plastic bag. Add steak sauce mixture; turn to evenly coat steak on both sides. Cover and refrigerate 1 hour to marinate, turning occasionally.
PREHEAT grill to medium-high heat. Remove steak from marinade; discard marinade.
GRILL steak 5 to 7 min. on each side or until cooked through, brushing occasionally with the remaining 1/3 cup steak sauce.