How to Make It
Light the grill or heat the broiler. Coat the steak with 1 tablespoon of the oil. Sprinkle the steak with 1/2 teaspoon salt and 1/4 teaspoon pepper. Grill or broil the steak for 5 minutes. Turn the meat and cook to your taste, about 5 minutes longer for medium rare. Transfer the steak to a carving board and leave to rest in a warm spot for 5 minutes.
Meanwhile, put the arugula on a platter or on individual plates. Top with the cherry tomatoes and sprinkle with 1/8 teaspoon salt.
In a medium glass or stainless-steel bowl, whisk together the mustard, anchovy paste, vinegar, capers, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Add the remaining 5 tablespoons oil slowly, whisking.
Slice the steak on the diagonal. Put the slices on top of the arugula and tomatoes and drizzle the vinaigrette over all.
Variation Use two quarts of watercress or spinach leaves instead of the arugula.