Place steak in casserole dish. Combine soup, vinegar, oil, garlic and pepper in medium bowl. Pour half of soup mixture over steak, coating both sides. Refrigerate remaining soup mixture. Seal bag; marinate in refrigerator 3 hours or overnight.

To complete recipe, preheat broiler and line broiler pan with aluminum foil. Place steak on pan, discarding marinade. Broil 13 to 18 minutes for medium rare to medium or until desired doneness, turning once. Remove steak to cutting board and let rest 5 minutes.

Place onion slices on broiler pan. Broil 2 minutes per side or until tender; remove from heat.

While onion slices are broiling, bring reserved soup mixture to a boil in small saucepan over medium-high heat. Reduce heat to medium-low and simmer 2 minutes.

Cut steak across the grain into thin slices. Place steak and onion on rolls. Serve sandwiches with extra sauce for dipping.