Tie the roast with cooking twine between the rib bones (in three sections) so it will hold its shape while roasting, and allow it to sit at room temperature for 1 hour.

Preheat the oven to 400°F.

Place the olive oil, garlic, anchovies, chopped onion, rosemary, salt, and pepper in the bowl of a food processor and process until you have a smooth paste. (Alternatively, chop everything except the oil together, then mash the mixture in a mortar and pestle until smooth; then incorporate the oil.) Set the paste aside.

Place the onion, carrot, and celery pieces in a roasting pan or a metal baking dish that is large enough to hold the rib roast. Using a flexible spatula, smear the paste on all sides of the roast. Set the roast on top of the vegetables and roast in the oven for 1 1/2 hours.

Continue to cook to desired degree of doneness, checking the internal temperature of the roast with an instant-read thermometer inserted in the thickest part. It should read 125°F to 130°F for rare to medium- rare. If you would prefer the roast to be medium to medium-well throughout, add a cup of broth to the pan to prevent the vegetables from burning, and continue to cook up to 30 minutes longer.

Remove the roast from the oven and cut the string. Brush the gremolata over the hot roast, and allow the roast to rest for 30 to 40 minutes so that the juices can redistribute through the meat.

To carve the roast, detach the entire section of rib bones by slicing along the side of the roast against the bone until it is separated. Turn the roast on its flat side and slice across the grain to the desired thickness. Alternatively, you can serve the beef without separating the ribs: turn the roast on its end (vertically), and while holding it steady with a carving fork, carve slices by cutting against the grain with a very sharp knife. Or (probably the easiest way to envision slicing), lay the roast on a cutting board, ribs down, and carve by slicing the ribs apart.

Serve the sliced roast with warm Fresh Horseradish Cream Sauce.