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1
Place a rack in the middle of the oven and preheat to 400 degrees F.
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2
Place the potatoes in a large roasting pan and toss with 1 tablespoon of the oil, the rosemary, and salt and pepper to taste.
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3
Roast for 30 minutes, tossing them once or twice, or until beginning to brown, but not cooked through.
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4
Remove and set aside.
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5
While the potatoes are roasting, place the horseradish in a fine-mesh strainer and drain off any liquid.
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6
Season the beef on all sides with salt and pepper and the thyme.
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7
In a large skillet, heat 2 tablespoons of the oil over high heat.
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8
Sear the meat on all sides, about 2 minutes per side, reducing the heat if the meat starts to burn.
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9
Remove from the heat and let cool thoroughly.
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10
(The meat and potatoes can be prepared several hours ahead of time up to this point.
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11
Cover and refrigerate until ready to roast.
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12
Be sure to remove the meat 15 to 20 minutes before roasting to bring the meat to room temperature.)
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13
Preheat the oven to 425 degrees F. Place the meat on a roasting rack inside the roasting pan.
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14
The potatoes can either go underneath the beef (they will be basted with the beef juices) or you can push them off to the sides.
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15
Pat the mustard on top of the tenderloin, and then top with the drained horseradish, pressing down to form a crust.
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16
Season with salt and pepper.
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17
Drizzle the remaining tablespoon of oil on top of the horseradish crust.
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18
Place the garlic next to the potatoes.
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19
Roast the beef and potatoes on the middle shelf for 45 minutes to 1 hour, or until the internal temperature of the meat is 140 degrees F for medium-rare.
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20
(The temperature will continue to rise once you take the meat out of the oven.)
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21
Let sit for 5 minutes, loosely covered with foil, before carving.
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22
The garlic can be served whole or squeezed out of the skins and served alongside the meat and potatoes.
