Preheat oven to 450 degrees Fahrenheit. Lay prime rib roast bone-side down in a large roasting pan.

In the bowl of a food processor, place garlic (the full one and a half cups if roast is larger), horseradish, rosemary, thyme, salt, and pepper. Pulse until garlic is minced and mixture is beginning to come together. Drizzle olive oil through the open spout as the processor is going until all the ingredients are combined and form a paste. Depending on the amount of garlic, you may not need the full cup of oil.

Generously spoon paste over entire surface of roast, using a spoon or your hands to rub it all over the bottom and sides as well.

Place roasting pan in the oven, uncovered, and bake for 20 minutes. Reduce heat to 350 degrees Fahrenheit for about another one and a half to two hours, depending on size of roast, for medium-rare results. (The general rule for prime rib roast is 15 minutes cook time per pound for rare to medium-rare results.  If you want it more well-done, cook 20 minutes per pound, but you shouldn’t let it cook longer than that. You can also turn oven off and leave roast inside for an extra half hour to continue cooking gently without drying it out.) Wait 20 minutes before carving to allow meat juices to rest and redistribute.

Serve with reserved juices from pan and garnish with fresh rosemary.

If making ahead of time, undercook by 10 to 20 minutes, then remove from fridge several hours before serving to allow meat to come to room temp. Either reheat at 350 degrees Fahrenheit for 30 minutes or 190 to 200 degrees Fahrenheit for two hours. Lightly cover with foil, forming a tent over the meat rather than tightly sealing it. Remove from oven 20 minutes before carving and serving. Do not slice the roast ahead of time and reheat like that, it will totally lose its juiciness and flavor.