Arrange steak and onion in 13 x 9-inch glass baking dish or large resealable plastic bag. Add 1/3 cup Wish-Bone® Italian Dressing; turn to coat. Cover, or close bag and marinate in refrigerator 30 minutes.
Remove steak and onion from Dressing, discarding Dressing. Grill or broil steak and onion, turning occasionally and brushing steak and onions with remaining 1/3 cup Dressing, until steak is desired doneness and onion is tender.
Let steak stand 10 minutes, then thinly slice. To serve, arrange sliced steak on serving platter. Separate onion into rings, then sprinkle over sliced steak.