How to Make It
Preheat the oven to 350°. Spread the butter over the cut side of the garlic and wrap it loosely in foil, crimping the package to seal. Roast for about 45 minutes, or until very soft. Squeeze the soft garlic cloves from the skin and strain into a small bowl. Cover with plastic wrap.
Increase the oven temperature to 400°. Put the reserved rib bones in a roasting pan and roast for 30 minutes, or until browned. (Leave the oven on.) Transfer the rib bones to a medium saucepan, cover with 4 cups of water and add 1/2 teaspoon of salt. Cover partially and simmer over low heat until the liquid is reduced to 1 cup, about 1 1/2 hours. Strain the stock into a bowl.
Meanwhile, in a spice grinder, grind the peppercorns, coriander, cumin and anise seeds to a coarse powder. Rub the spice mix all over the roast and season it with salt. Set the meat on a rack in a roasting pan and roast in the center of the oven for 15 minutes. Reduce the temperature to 325° and roast for about 2 hours longer, or until an instant-read thermometer inserted in the center of the roast registers 120° for medium rare. Transfer the roast to a carving board. Cover loosely with foil and let rest for 10 to 15 minutes before slicing.
Pour the juices from the roasting pan into a glass measure and discard the fat. Set the roasting pan over two burners on moderately high heat. When the pan starts to sizzle, add 1/2 cup of water and simmer, scraping the bottom of the pan to loosen any browned bits. Stir in the reserved stock and simmer over low heat for a few minutes. Pour this jus into a small saucepan and add the reserved pan juices from the glass measure; keep the jus warm over very low heat.
Using a large, sharp knife, carve the roast into 1/2-inch-thick slices; add any meat juices to the jus. Whisk in 1 tablespoon of the roasted-garlic puree. Season the jus with salt and pepper and serve with the roast.