Preheat the oven to 425 degrees F.
For the sauce: In a medium bowl, combine the sour cream, mayonnaise, horseradish, mustard, salt, pepper and chives. Mix well. Let sit in refrigerator for at least 20 minutes before serving. (Can be made a day ahead.)
For the tenderloin: Place the parsley, rosemary, thyme, garlic and salt in a food processor. Pulse until finely chopped. Add 1/4 cup olive oil and pulse a few times to make a thick paste.
Heat a large skillet over high heat. Lightly brush the tenderloin with olive oil and sprinkle with black pepper. Sear on all sides, about 2 minutes per side. Rub the tenderloin with the herb mixture.
Place the pan into the oven; roast until the tenderloin reaches an internal temperature of 135 degrees F for medium, 35 to 40 minutes.
Remove from the oven, cover with foil and let rest for 10 minutes. Remove the string, slice, and serve with horseradish cream sauce.
Recipe courtesy of Tia Mowry