Trim fat from steak. Using a sharp knife, score steak by making shallow diagonal cuts at 1-inch intervals in a diamond pattern. Repeat on other side. Place steak in a plastic bag set in a shallow dish.

For marinade, in a small bowl combine 2 tablespoons of the mustard, the lemon juice, and Worcestershire sauce. Pour over steak; seal bag. Marinate in the refrigerator for 6 to 24 hours, turning bag occasionally.

For sauce, in a small bowl combine the remaining mustard, the sour cream, green onion, and horseradish. Cover and refrigerate. Remove from refrigerator about 30 minutes before ready to serve.

Drain steak, discarding marinade. Grill steak on the rack of an uncovered grill directly over medium coals until desired doneness, turning once halfway through grilling. (Allow 12 to 14 minutes for medium doneness.) [Or, broil on the unheated rack of a broiler pan 3 to 4 inches from the heat, turning once halfway through broiling. (Allow 12 to 14 minutes for medium doneness.)]

To serve, thinly slice steak across the grain. Serve with the sauce.