Heat oven to 400 degrees. Pat beef dry with paper towel. Heat oil in a medium size, flameproof roasting pan set over 2 stovetop burners over medium-high heat. Brown beef well on all sides, about 10 minutes total. Remove beef to a cutting board and let cool; discard oil from roasting pan.
Meanwhile mash garlic with salt in a mortar with pestle (or scrape mixture together on a cutting board with the flat side of a chef's knife), until a fine paste forms. Stir in thyme, rosemary and pepper. Rub mixture all over beef; place beef on a rack in a roasting pan.
Roast 25 to 30 minutes, until an instant-read thermometer registers 125 degrees in the thickest part.
Transfer beef to a cutting board and let stand, loosely covered with foil, 15 to 20 minutes (it will continue to cook, reaching 130 degrees in the thick end for medium rare, and the narrow end at 135 degrees for medium).
Remove string from beef and discard. Cut beef into 1/2 inch thick slices. Serve with Horseradish Chive Sauce.
In a bowl combine horseradish sauce, sour cream, heavy cream, dijon mustard, horseradish, lemon juice, salt and pepper until blended. Refrigerate (can be made up to 2 days ahead). Just before serving, stir in chives.