How to Make It
Put the allspice berries in a spice grinder and finely grind. Pass through a fine strainer into a bowl. Put the coarse bits in the strainer back in the grinder. Add the peppercorns and coarsely grind. Transfer to the bowl. Coarsely grind the mustard seeds and add to the bowl. Coarsely grind the coriander and then the cumin seeds; add to the bowl. Stir the 3 tablespoons of salt into the spices. Rub the spice blend all over the meat. Cover the roast and refrigerate overnight.
Put a large roasting pan in the middle of the oven and preheat the oven to 375°. Let the rib roast stand at room temperature for 30 minutes.
Season the roast with salt and place it in the hot roasting pan, fat side down. Roast for 30 minutes. Turn the roast over and roast at 350° for about 3 hours, rotating the pan 2 or 3 times. The roast is done when an instant-read thermometer inserted in the thickest part of the meat registers 130° for medium-rare. Transfer the roast to a carving board to rest for about 20 minutes.
Meanwhile, in a bowl, stir the yogurt with the horseradish, ginger, black pepper and sugar and season with salt. Refrigerate the raita until 20 minutes before serving.
Carve the roast and serve with the raita.