Preheat the oven to 450 degrees. Put the oxtail in a high-sided roasting pan and roast in the oven for 30 minutes, turning it once.  Remove it from the oven and ppour off the rendered fat.

Distribute all the vegetables and spices - including the cumin, coriander seed, bay leaves, and capers - around the oxtail. Pour the tomatoes and beef stock over the oxtail with enough water to cover.

Place in the hot oven for 1 hour.

Check every 15 to 20 minutes and stir with a spoon to break up any crust that forms. After 1 hour reduce the heat to 350 degrees and continue cooking until the oxtail is done, about 1 hour more. Pull one large ppiece of oxtail to check for doneness: The meat should fall from the bone.

Remove the oxtail from the sauce and skim off as much fat as you can. Let the oxtail cool and chill overnight in a container, covered with the sauce. When cold, remove the layer of solidified fat on the surface. To serve, slowly reheat the oxtail in the sauce and stir in the chopped cilantro.