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 unpeeled 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 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 the spice mix over the roast.
- Preheat oven to 450 degrees F (230 degrees C). Place the celery, carrot, and onion pieces into 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 (175 degrees C), 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 (54 to 57 degrees C) for medium-rare. 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 1 1/2 cup 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, and whisk the mixture into the sauce. Allow the sauce to thicken slightly (sauce will be thin), pour into a gravy boat, and serve with roast.