How to Make It
Preheat oven to 425°F. Stir together horseradish, mustard, parsley, garlic, and salt in a medium bowl. Rub mixture on beef tenderloin, pressing to adhere, and place on a rimmed baking sheet. Bake in preheated oven until a thermometer inserted in thickest portion of beef registers 135°F for medium-rare, about 30 minutes. Remove to a cutting board, and let stand 15 minutes before slicing.