1 (7 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 Includes Leaf
1 Carrot, Cut Into 2-Inch Pieces
1 Medium Unpeeled Onion, Quartered And Separated
2 Teaspoons Concentrated Beef Base (paste)
1 ½ Cups Water
1 Teaspoon Cornstarch
1 Teaspoon Water
Step 1 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.
Step 2 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.
Step 3 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.
Step 4 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. Serve with green beans, garlic mash, rolls and beverage.