1) First, make a marinade for the bull's or ox tails. In a large deep tray or pan, place the bull's or oxtails, in a single layer, standing upright. Press a couple of saffron threads into the top of each tail. Add the chopped vegetables to the marinade and finally, pour in one bottle or more if needed, of Spanish red wine. The red wine should just cover all the ingredients. Leave to marinate in the refrigerator for 24 hours.
2) To make the stew, remove the bull's tails or oxtails and set aside. Strain the vegetables from the marinade and reserve both the liquid and the strained vegetables.
3) In a Dutch or deep, heavy skillet, heat 115ml of olive oil, and render down the ham fat, if using. Strain out the marinade and add the reserved strained vegetables and saute; until lightly caramelised. Add bell pepper and continue to slowly caramelise all the ingredients together for 15 minutes longer.
Add the tails one by one, standing them upright and moving aside the caramelised vegetables underneath, so that the tails have direct contact with the bottom of the pan or pot. When the underside of the tails has turned a gray color, turn them over and repeat on the other side. Add in the reserved red wine from the marinade. The liquid should just barely cover the bull's or oxtails. If needed, top up the liquid with water. Add tomato puree and demi-glace, if using.
4) Cook over slow, low heat for four hours until the meat from the tails is falling off the bone but still succulent and moist. Check and correct seasoning of the stew with salt and freshly ground pepper. Serve the stew immediately, garnished with finely chopped parsley, accompanied by roasted potatoes and crusty bread. This stew can also be made the day ahead and reheated to serve the following day.