- 1 (6-pound) boneless prime rib roast
- 2 tablespoons prepared horseradish
- 2 tablespoons Dijon mustard
- 2 teaspoons kosher salt
- 2 teaspoons coarsely ground black pepper
- 2 teaspoons dried thyme
- 2 teaspoons garlic powder
- 2 stalks celery, cut into 2-inch pieces
- 1 carrot, cut into 2-inch pieces
- 1 small onion, quartered and separated
- 2 teaspoons concentrated beef base (paste)
- 1-1/2 cups water
- 1 teaspoon cornstarch
- 1 teaspoon water
- The day before serving, remove the roast from the package and dry thoroughly with paper towels. Set the roast on a baking sheet and place it in refrigerator overnight.
- Remove from refrigerator 1 hour before cooking time to allow meat to reach room temperature.
- Rub the roast all over with horseradish and Dijon mustard. In a bowl, mix together the kosher salt, black pepper, thyme and garlic powder; sprinkle this spice mix over the roast.
- Preheat oven to 450 degrees F.
- Place the celery, carrot and onion pieces in the bottom of a roasting pan. Place the roast on top of the vegetables. Roast in the preheated oven for 30 minutes.
- Reduce oven temperature to 350 degrees F, and roast until the meat is browned and an instant-read meat thermometer inserted into the thickest part of the roast reads 130 to 135 degrees F for medium-rare, between 90 minutes and 2 hours.
- Remove from oven, transfer roast to a platter, and cover with a tent of aluminum foil. Allow to rest for 30 minutes. Temperature of the meat will rise about 10 degrees during resting time.
- To make au jus sauce, skim excess fat from the pan drippings in the roasting pan. Place the pan over a burner set to medium heat, and stir in the beef base and 1½ cups of water.
- Bring to a boil, scraping and dissolving any brown flavor bits from the bottom of the pan. Strain out and discard the vegetables.
- Combine the cornstarch and 1 teaspoon of water in a small bowl until it forms a slurry, and whisk this mixture into the sauce. Allow the sauce to thicken slightly (sauce will be thin), pour into a gravy boat, and serve with roast.