Bring the prime rib to room temp by removing it from the refrigerator for at least 1 hour before you cook it.
In a small bowl, combine the Dijon mustard and the horseradish sauce. Mix until fully combined.
Brush the sauce onto the prime rib just before you are about to cook it.
Sprinkle on the remaining seasonings. Do not cover the prime rib when you cook it.
Cook it at 450 to 500 degrees (some ovens only go up to 450 degrees) for 5 minutes per pound of prime rib. Then turn off the oven and allow the roast to continue cooking for 2 hours. Don't open the oven or else you will lose the heat.
Read all the tips in the post above for all the cooking tips!
Slice and serve.
How to make Au Jus from prime ribYou want to scrape up the little brown bits from the bottom of the roasting pan and use those for your sauce. Those brown bits are loaded with flavor! Add 1 cup of beef broth to these brown bits. Then salt and pepper to taste. Bring to a simmer. Serve along with the sliced prime rib.