Heat half the oil in a large Le Creuset style oven-proof casserole dish and add the oxtail. Brown the oxtail until dark brown on all sides, this will take at least ten minutes. Remove the meat from the pan and set aside.
Preheat your oven to 150°C / Fan 130°C / Gas 2.
Pour the rest of the oil into the same pan together with the onions, carrots, celery and garlic. Stirring every now and again let the veggies cook for about ten minutes until softened.
Tip the oxtail back into the pan together with all the other ingredients. Stir well and bring to a simmer.
Place the lid on the pan and put in your preheated oven. Leave the stew to cook for about three hours, checking once or twice during this time and giving everything a good stir. If it looks a little dry, stir in a splash of water or more stock.
The stew’s ready when the meat’s starting to fall off the bone.