Cold Beef With Mustard Horseradish Sauce(Prep Time:  15 Minutes.)  
["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."]
["red wine vinegar", "coarse-grained mustard", "horseradish", "scallions", "watercress", "mayonnaise", "beef"]