Grilled Flank Steak Sandwiches with Horseradish Sauce

1 beef flank steak (about 1 pound)
2 tablespoons reduced-sodium soy sauce
1 tablespoon red wine vinegar
2 cloves garlic,minced
1/2 teaspoon black pepper
1 cup fat-free sour cream
1/4 cup finely chopped fresh parsley
1 tablespoon prepared horseradish
1 tablespoon Dijon mustard
1/2 teaspoon salt
6 sourdough rolls,split
6 romaine lettuce leaves

1.Place flank steak in large resealable food storage bag.Add soy sauce,vinegar,garlic and pepper.Seal bag;turn to coat.Marinate in refrigerator at least 1 hour.
2.Prepare grill for direct cooking.Drain steak;discard marinade.Grill steak over medium heat,uncovered,17 to 21 minutes for medium-rare to medium or until desired doneness,turning once.Cover with foil;let stand 15 minutes.Thinly slice steak across the grain.
3.Combine sour cream,parsley,horseradish,mustard and salt in small bowl until well blended.Spread rolls with horseradish sauce;layer with sliced steak and lettuce.
(makes 6 servings)