Prepare the Prime Rib Roast: Remove the prime rib from the refrigerator at least 2 to 3 hours before you plan to cook it. This is crucial for even cooking. Pat the roast completely dry with paper towels. Drying the surface helps to create a beautiful, browned crust.

Preheat the Oven: Preheat your oven to 230°C (450°F). Ensure your oven rack is in the lower-middle position.

Prepare the Seasoning Rub: In a small bowl, combine the kosher salt, freshly ground black pepper, garlic powder, onion powder, dried rosemary, and dried thyme. Mix thoroughly.

Coat the Roast: Drizzle the olive oil all over the prime rib roast. Use your hands to rub it evenly across the entire surface of the meat. Then, generously sprinkle and rub the prepared seasoning mixture all over the roast, ensuring it's well-coated on all sides, including the ends.

Place Roast in Pan: Place the seasoned prime rib roast, bone-side down, into a sturdy roasting pan. If your pan has a rack, place the roast on the rack, bones acting as a natural support. This helps air circulate for a more even cook.

Initial High-Heat Sear: Place the roasting pan with the prime rib into the preheated oven. Sear the roast at 230°C (450°F) for 15 minutes. This initial blast of high heat helps to develop a delicious brown crust.

Reduce Oven Temperature and Continue Roasting: After the initial 15 minutes, reduce the oven temperature to 160°C (325°F) without opening the oven door unnecessarily. Continue roasting the prime rib. The general guideline for roasting is approximately 13-15 minutes per pound (or about 28-33 minutes per kilogram) for medium-rare, but it's best to use a meat thermometer for accuracy.

Monitor Internal Temperature: Start checking the internal temperature about halfway through the estimated cooking time. Use an instant-read meat thermometer inserted into the thickest part of the roast, avoiding the bone. The target temperature for medium-rare is 52°C (125°F) and for medium is 57°C (135°F). Remember the temperature will rise by about 5-10°C (10-15°F) as it rests.

Rest the Roast: Once the desired internal temperature is reached (remembering it will rise during resting), carefully remove the prime rib from the oven. Transfer the roast to a clean cutting board. Tent it loosely with aluminum foil and let it rest for at least 20 to 30 minutes. This resting period is critical for allowing 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, mayonnaise, prepared horseradish, Dijon mustard, fresh lemon juice, and Worcestershire sauce. Stir until well combined and smooth.

Season the Sauce: Season the horseradish sauce with salt and freshly ground black pepper to taste. If you prefer a spicier sauce, you can add a little more prepared horseradish. If it's too thick, you can thin it with a teaspoon or two of milk or cream.

Carve and Serve: After resting, remove the foil. You can either carve the roast directly from the bone or remove the bones first (if you are comfortable doing so) and then carve. Slice the prime rib into desired thickness, about 1.25cm (1/2 inch) thick. Serve immediately with the creamy horseradish sauce on the side. Spoon any pan juices over the sliced meat if desired.