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° F.
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 preheated oven for 30 minutes.
Reduce oven temperature to 350° and roast until the meat is browned and an instant-read thermometer inserted into the thickest part of the roast reads 130° to 135° 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 - Au Jus
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 ½ 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 tsp water in a small bowl and whisk the mixture into the sauce.
Allow the sauce to thicken slightly, pour into gravy boat and serve with roast.