Preheat the oven to 450 degrees F. Set the roast in the centre of a very large roasting pan; season with salt and pepper. Roast for 15 minutes. Meanwhile place the carrots, parsnips, onion, potatoes, oil, rosemary, garlic, salt and pepper in a bowl and toss to combine. When the prime rib has roasted 15 minutes, surround it with the vegetables. Reduce the oven temperature to 325 degrees F. For a medium rare prime rib, roast 60 to 70 minutes more, or until the centre of the roast reaches 130 degrees F on an instant read meat thermometer. When the beef is ready, transfer to a plate, tent with foil and allow to rest for 10 to 15 minutes. Transfer the vegetables to a heatproof serving dish and keep warm in a 200°F oven. Thinly slice the beef and serve the vegetables alongside. Serve with horseradish, if desired.

Note: To make jus to serve with the roast, remove excess fat from the roasting pan, pour in 3 cups of beef stock, and simmer 5 minutes.