Roast Beef Baguettes

1. Combine vinegar and onion in a bowl. Let stand, stirring once or twice, 15 minutes; drain. 

2. Combine butter, horseradish, and mustard in a bowl. Spread butter mixture onto bottom half of baguette. Top with roast beef, cress, and pickled onions. Sandwich with remaining bread. Cut into 6 pieces.