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.