1/2 cup sour cream
1 tablespoon plus 1 teaspoon prepared horseradish
2 green onions, chopped
Romaine lettuce leaves
FOR STEAK: Combine first 6 ingredients in glass baking dish. Add steak and turn to coat. Cover and marinate overnight, turning occasionally.
FOR SAUCE: Mix first 3 ingredients in small bowl. Season with generous amount of pepper. (Can be prepared 1 day ahead. Cover and refrigerate.)
Prepare barbecue (high heat). Drain steak. Pour marinade into small saucepan and boil 1 minute. Season steak with generous amount of pepper. Grill to desired doneness, basting occasionally with marinade, about 6 minutes per side for rare. Transfer steak to plate and let stand at least 15 minutes. (Can be prepared up to 2 hours ahead.) Cut steak across grain into thin diagonal slices. Line platter with romaine leaves. Top with steak. Serve warm or at room temperature with horseradish sauce.