Put oven rack in middle position and preheat oven to 400F.
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.
Roast tenderloin:
Increase oven temperature to 475F.
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 425F and cook until thermometer inserted diagonally into center of meat registers 130F, 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.
Sequencing the Oven Temperatures
Managing two distinct oven temperatures is the primary logistical hurdle when preparing center-cut beef tenderloin. Roasting requires an initial blast at 475 degrees Fahrenheit to set a robust crust on the exterior spices, followed by a drop to 425 degrees to finish the interior. Because whole tenderloin tapers at the ends, consider tying the roast with butcher's twine at even intervals to promote uniform thickness and prevent the skinny tail from overcooking before the center reaches 130 degrees for medium-rare.
Reducing the Cream and Roasting Garlic
Heavy cream simmers down from two cups to about three-quarters of a cup over twenty to twenty-five minutes, thickening naturally without requiring flour or additional starches. Whole heads of garlic require significantly longer at one and a quarter hours in a 400-degree oven, meaning that bulb must go into the heat well before the meat begins cooking. Squeezing the softened cloves from their papery skins yields a sweet, mellow paste that blends smoothly into the reduced dairy and drained horseradish without imparting harsh raw pungency.
Avoiding Common Pitfalls
Overcooking remains the single greatest risk with lean cuts like beef tenderloin, which lacks the internal fat marbling of ribeye or chuck. Pulling the meat from the oven when an instant-read thermometer registers 130 degrees allows residual carryover heat to finish the cooking process during the mandatory ten-minute resting window on a rack. Skipping that rest causes valuable juices to bleed onto the carving board rather than remaining inside the muscle fibers.