2 tbsp. red wine vinegar
2 tbsp. coarse-grained mustard
1 tbsp. bottled horseradish, drained
2 scallions, minced
1/4 minced watercress
1/2 c. mayonnaise
1 1/2 lb. rare roast beef or leftover steak (at room temperature), thinly sliced

In a small bowl, whisk together the vinegar, mustard, horseradish, salt and pepper. Whisk in the oil until emulsified. Whisk in the scallions, watercress and mayonnaise. Arrange the beef on a platter and serve it with mustard sauce.