Pound steak on both sides with a wooden mallet or the edge of a heavy plate.
Combine lemon juice and mustard and spread over both sides of steak. Allow to stand 4 to 5 hours at room temperature.
Broil steaks about 4 inches above glowing coals on barbecue grill for 4 to 5 minutes on each side, or until desired degree of doneness is achieved.
You won't taste the lemon juice or mustard as such - just a very good and flavorful steak.