Ingredients

• 2 tablespoons red-wine vinegar
• 2 tablespoons coarse-grained mustard
• 1 tablespoon bottled horseradish, drained
• 2 scallions, minced
• 1/4 minced watercress
• 1/2 cup mayonnaise
• 1 1/2 pounds rare roast beef or leftover steak at room temperature, thinly sliced

Instructions

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