Season the prime rib liberally on all sides with salt and black pepper.

Line a roasting pan with heavy-duty aluminum foil and position a roasting rack inside of it.

Transfer the prime rib fat side up to the middle of the roasting rack. Tent the prime rib loosely with foil, and allow it to stand at room temperature for 1-2 hours.

Preheat the oven to 500°F.

Place the Dijon mustard, horseradish, garlic powder, thyme, rosemary, and onion powder into a small bowl. Stir to combine.

Remove the foil tent from the roast. Using the back of a spoon, spread the mustard mixture over the top surface of the meat.

Roast the prime rib on middle rack of oven for 20 minutes.

Reduce the oven temperature to 325°F. Continue roasting the prime rib until the meat reaches an internal temperature of 120-145°F, depending on desired doneness, 75-100 minutes.

Check to see that prime rib is done. Remove from oven or add time as needed.

Allow the prime rib to rest on the rack for 15-30 minutes, then transfer to a cutting board and untie. Carve and serve immediately. Refrigerate leftovers up to 5 days or freeze up to 6 months.