Special equipment: a roasting pan with a rack; an instant-read thermometer
Put oven rack in middle position and preheat oven to 400°F.
Cut off and discard top fourth of garlic head. Drizzle exposed garlic with oil and sprinkle with 1/4 teaspoon salt, then wrap head tightly in foil. Roast garlic until tender, about 1 1/4 hours, then open foil and cool.
While garlic roasts, simmer cream in a 1 1/2- to 2-quart heavy saucepan, stirring occasionally, until reduced to about 3/4 cup, 20 to 25 minutes, then transfer to a bowl.
Squeeze garlic into a small bowl, discarding skins, and mash together with horseradish, pepper, and remaining 1/2 teaspoon salt using a fork.
Stir garlic mixture into cream, then chill until ready to use.
Increase oven temperature to 475°F.
Pat tenderloin dry. Stir together pepper, bouillon, salt, cornstarch, oregano, garlic powder, and paprika in a small bowl. Rub oil all over tenderloin, then sprinkle with spice mixture, rubbing it into meat. Put tenderloin on rack in roasting pan and roast 10 minutes, then reduce oven temperature to 425°F and cook until thermometer inserted diagonally into center of meat registers 130°F, 20 to 25 minutes for medium-rare. Let beef stand on rack in pan 10 minutes before serving.
Cut beef into 1/4-inch-thick slices and serve with garlic horseradish cream.