Trim fat from beef. Use a sharp knife to score beef by making shallow diagonal cuts at 1-inch intervals in a diamond pattern. Repeat on other side. Place beef in a plastic bag set in a shallow dish.
For marinade, in a small bowl combine 2 tablespoons of the Dijon-style mustard, the lemon juice, and Worcestershire sauce. Pour over beef. Close the bag. Marinate beef in the refrigerator for 6 to 24 hours, turning the bag occasionally.
In a small bowl combine the remaining Dijon-style mustard; the sour cream, mayonnaise, or salad dressing; green onion; and horseradish. Cover and refrigerate. Remove from refrigerator 30 minutes before serving time.
Drain the beef, discarding the marinade. Grill on the rack of an uncovered grill directly over medium coals to desired doneness, turning once. Allow 12 to 14 minutes for medium.
To serve, thinly slice beef across the grain. Serve with sour cream mixture. Makes 4 servings.