This garlicky roast cooks for around 3 hours, meaning you'll get delicious flavor and great smells in your home.
Preheat the oven to 350°. In a food processor, puree the first 5 ingredients. Place the roast, rib-side down, in a large roasting pan. Score the fat in a 1-inch crosshatch pattern. Season liberally with salt and pepper. Smear the horseradish on the roast. Roast until an instant-read thermometer inserted into the thickest part of the roast registers 120° for medium-rare, 2 1/2 to 3 hours.
Let the roast rest in the pan for 20 to 30 minutes. Transfer to a platter. Spoon the fat off the drippings in the pan. Place the pan over 2 burners on the stovetop. Stir the drippings over high heat until warmed, then pour into a pitcher. Carve the roast. Serve with the drippings.
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