Place the beef on a baking sheet and pat the outside dry with a paper towel. Spread the butter on with your hands. Sprinkle evenly with the salt and pepper. Roast in the oven for exactly 22 minutes for rare and 25 minutes for medium rare.
Remove the beef from the oven, cover it tightly with aluminum foil, and allow it to rest at room temperature for 20 minutes. Remove the strings and slice the filet thick, 1/2-inch thick per slice. Transfer to a serving platter. Garnish with parsley. Serve warm, at room temperature, or cold with Mustard Horseradish Sauce on the side.
Mustard Horseradish Sauce
Whisk together the mayonnaise, sour cream, mustards, horseradish, and salt in a bowl. Refrigerate until ready to serve.