Prepare the Prime Rib Roast: Remove the prime rib roast from the refrigerator at least 2-3 hours before cooking to bring it to room temperature. This is crucial for even cooking. Pat the roast completely dry with paper towels. This helps create a beautiful, flavorful crust.

Preheat the Oven: Preheat your oven to 240°C (475°F). Ensure the oven rack is in the lower third of the oven.

Create the Rub: In a small bowl, combine the kosher salt, freshly ground black pepper, garlic powder, dried rosemary, and dried thyme. Mix well to ensure all spices are evenly distributed.

Season the Roast: Drizzle the olive oil all over the prime rib roast and rub it in to coat the entire surface. Generously sprinkle the spice rub all over the roast, pressing it gently into the meat to adhere. Ensure every surface is coated.

Roast at High Temperature: Place the seasoned prime rib roast, fat-side up, in a sturdy roasting pan. If you have a V-rack, use it; otherwise, place it directly in the pan. Place the roast in the preheated oven.

Reduce Oven Temperature: After 15 minutes at 240°C (475°F), reduce the oven temperature to 160°C (325°F) without opening the oven door for too long. Continue roasting.

Calculate Roasting Time: The general guideline for roasting prime rib is approximately 13-15 minutes per pound (or about 28-33 minutes per kilogram) for medium-rare. For a 2.5kg (5.5 lb) roast, this would be roughly 70-85 minutes of cooking time after reducing the temperature. Target Internal Temperatures: - Rare: 50-52°C (122-125°F) - Medium-Rare: 55-57°C (130-135°F) (Recommended) - Medium: 60-63°C (140-145°F) - Medium-Well: 65-68°C (150-155°F) Use an instant-read meat thermometer inserted into the thickest part of the roast, avoiding bone, to check the temperature. Remember the temperature will rise 5-7°C (10-15°F) as it rests.

Check for Doneness: Start checking the internal temperature about 20 minutes before the estimated end of the cooking time. For medium-rare, aim for an internal temperature of 52-54°C (125-130°F) before removing it from the oven.

Rest the Roast: Once the desired internal temperature is reached, carefully remove the prime rib roast from the oven. Transfer it to a cutting board and tent it loosely with aluminum foil. Let it rest for at least 20-30 minutes (or up to 45 minutes for larger roasts). This resting period is critical for the juices to redistribute throughout the meat, ensuring a tender and moist roast.

Make the Creamy Horseradish Sauce: While the roast is resting, prepare the sauce. In a medium bowl, combine the sour cream, prepared horseradish, and Dijon mustard. Stir until well combined and smooth. Taste and adjust seasoning, adding more horseradish for a spicier kick or a pinch of salt if needed. Cover and refrigerate until ready to serve.

Prepare Au Jus (Optional): If you wish to make an au jus, carefully pour the drippings from the roasting pan into a small saucepan. Skim off any excess fat. Add the beef broth to the drippings. Bring to a simmer over medium heat and cook for a few minutes until slightly reduced and flavorful. Season with salt and pepper to taste.

Carve and Serve: After resting, remove the foil. If the roast is bone-in, you can either carve between the bones or carefully remove the bones first by running a sharp knife along the rib bones, then carving the roast into thick slices of your desired thickness (about 2.5cm or 1 inch). Spoon any accumulated juices over the sliced meat. Garnish with fresh chopped parsley.

Serve: Serve the sliced prime rib immediately with the creamy horseradish sauce and the optional au jus on the side. Enjoy your magnificent roast!