Combine butter, salt and pepper. Using paper towels, pat tenderloin completely dry. Spread seasoned butter over fillet. Place meat on hot grill and close lid tightly. (Grill temperature should remain fairly steady, going no lower than 375°.) For rare to medium-rare, cook tenderloin approximately 30 minutes, turning once during grilling. (Internal temperature of thickest part of meat should read 125°.)
Meanwhile, to make sauce: Whisk together all mustard sauce ingredients.
When fillet is done, place on a cutting board and cover with aluminum foil; allow to rest about 20 minutes. Slice beef and serve with mustard sauce.
Recommended ingredients: Pommery mustard is available at specialty shops or online at www.frenchfeast.com.