Begin by tying the tenderloin at two-inch intervals to maintain a uniform shape. If there is a tapered end, then tuck it under and tie into place. Pat the tenderloin dry with paper towel.

In a small bowl combine the Dijon mustard, horseradish, chopped fresh rosemary, minced garlic, olive oil, kosher salt, and pepper. Stir together until fully combined.

Place a rack in a large roasting pan and drizzle the bottom with a little olive oil. Generously rub the marinade on all sides of the beef. Place the beef on the rack in the roasting pan. Let sit at room temperature for 45 minutes, covered if you prefer.

Preheat the oven to 425°F. Roast the beef for 35 to 45 minutes until it reaches your desired doneness. For medium rare, pull the roast when it reaches an internal temperature between 125°F and 130°F. For medium, pull the roast when it reaches 135°F. The temperature will continue to rise as the meat rests.

Remove the roast from the oven and transfer to a platter. Tent loosely with foil and let rest for 15 to 20 minutes.

In a small bowl combine the mayonnaise, sour cream, Dijon and grainy mustards, horseradish, parsley, chives, kosher salt and pepper. Whisk together until smooth. Chill until ready to serve.

To serve, use kitchen shears to snip and remove the twine. Carve the tenderloin into thick slices and arrange on a serving platter. Serve with the creamy mustard sauce and enjoy!