The day before serving, remove the roast from the package and dry thoroughly with paper towels. Set roast on a baking sheet, and place in refrigerator overnight. Remove from refrigerator 1 hour before cooking to allow meat to reach room temperature. Rub the roast all over with horseradish and Dijon mustard. Mix kosher salt, black pepper, thyme, and garlic powder together in small bowl; sprinkle over the roast.

Preheat the oven to 450 degrees F (230 degrees C). Place celery, carrot, and onion into the bottom of a roasting pan; place the roast on top.

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 degrees F (54 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. The temperature of the meat will rise about 10 degrees during resting time.

To make au jus sauce: Skim excess fat from pan drippings in the roasting pan. Place the pan over a burner set to medium heat; stir in beef base and 1 1/2 cups of water. Bring to a boil, scraping and dissolving any brown flavor bits from the bottom of the pan. Strain out and discard vegetables. Combine cornstarch and 1 teaspoon of water in a small bowl; whisk the mixture into sauce. Allow sauce to thicken slightly; pour into a gravy boat and serve with roast.