2. Using a sharp thin knife, trim any fat on the top of the roast to 1/4-inch thickness. Using a large knife, finely chop the garlic on a work surface. Sprinkle with 1/2 teaspoon of the salt and mash and smear the garlic on the work surface to make a paste. Scrape into a small bowl. Add the remaining 1 teaspoon salt, and the pepper, thyme, rosemary, and pepper. Rub the seasoning mixture all over the roast, including the underside.
3. Place the rib roast, meaty side up, in a large roasting pan. (No need for a roasting rack, as the bones create a natural one.) Roast for 15 minutes. Reduce the oven temperature to 350°F. (Do not open the oven door.) Continue roasting until an instant-read thermometer inserted in the center of the roast reads 115° to 120°F, about 2 hours total roasting time for medium-rare meat. For an accurate reading, be sure the tip of the thermometer is positioned in the center of the meat. Transfer the roast to a carving board. Let stand for 20 minutes.
4. While the meat is standing, make the "au jus" sauce: Pour out all of the fat from the roasting pan and discard (or reserve if making Yorkshire puddings). Place the roasting pan on two burners on medium heat. Add the beef stock and stir to release any browned bits in the pan. Bring to a boil and cook until the stock is slightly reduced, about 2 minutes. Transfer to a gravy boat.
4. To carve the roast, using a meat fork and a long thin carving knife, stand the roast on the its meaty end. In one piece, carve off the rib section, and set aside Slice the meat, cut the ribs into individual pieces, and serve immediately, with a spoonful of the au jus sauce poured over each serving. Pass the horseradish sauce on the side, if desired.
Sour Cream Horseradish Sauce: In a medium bowl, mix 1 pint sour cream, 1/2 cup prepared horseradish, well-drained, 1/4 cup chopped chives (optional), 1/2 teaspoon salt, and 1/4 teaspoon freshly ground pepper. If desired, add more drained horseradish to taste. Let stand at room temperature for 1 hour before serving.