Beef and Horseradish Sauce Sandwich

Name: Beef and Horseradish Sauce Sandwich
Ingredients: 2 to 3 pounds fillet of beef, trimmed and tied; 1 tablespoon unsalted butter, at room temperature; 1 teaspoon Dijon mustard; 2 teaspoons kosher salt; 2 teaspoons coarsely ground black pepper; 1 loaf health or 7-grain bread; Mustard Horseradish Sauce, recipe follows; 1 bunch arugula; Kosher salt; Freshly ground black pepper; Unsalted butter at room temperature; 3/4 cup good mayonnaise; 1 1/2 tablespoons Dijon mustard; 1 tablespoon whole-grain mustard; 1/2 tablespoon prepared horseradish; 2 tablespoons sour cream