Preheat the oven to 160ºC/gas mark 3. Season the oxtail with salt and black pepper.
Heat the oil in a large, flameproof casserole dish that has a lid (it needs to be able to fit 2 litres of liquid plus lots of veg and oxtail, so the bigger the better) and brown the oxtails in a couple of batches on all sides. You’ll need to spend 2–3 minutes on each side to get a good colour. Transfer each batch to a bowl and set aside and keep warm.
Add the carrots, celery, leek and onions to the pan with the thyme and bay leaves. Gently cook these for 10–12 minutes on a moderate heat until the onions have softened and are starting to brown at the edges. Now add the garlic and cook for a couple more minutes.
Pour in the wine, beer, stock, chopped tomatoes and some black pepper. Don’t add any salt yet as the sauce will be reduced later, which will intensify all the flavours.
Now return the browned oxtail and any collected juices to the pan. Bring the stew to simmering point and pop the dish in the oven with the lid on for 3 1/2 hours.
The oxtail will now be extremely tender; remove the pieces carefully from the pan and set aside on a plate. Sieve the sauce into a bowl using the back of a wooden spoon to push through all the liquid you can from the cooked vegetables. Discard the drained vegetables then return the sieved liquid to the dish and reduce it until it’s the consistency of single cream and rich in flavour.This may take up to 20 minutes. Add a little salt if you feel it needs it.
Meanwhile, with the point of a knife, make a cross at the base of each tomato. Put the tomatoes in a heatproof bowl and pour over boiling water until they are submerged, then leave them for 1 minute. Drain and cool them under cold water.The skins should now slip off. Cut the tomatoes into quarters and remove the seeds, then chop the flesh into small pieces.
Put all the finely chopped vegetables into a small saucepan, except the tomatoes. Add 2 tablespoons of water and cook on a high heat, with a lid on, for 3–4 minutes until the vegetables have softened slightly.
Once the meaty sauce has fully reduced and you’re about to serve the dish, add the cooked vegetables and the oxtail back to the sauce to heat through. Then gently mix in the peeled and chopped tomatoes. Serve with mashed potato and sprinkle with a little chopped parsley. You can make everything a day ahead up until the point of cooking the diced vegetables and preparing the tomatoes. Before serving, simply warm the stew through and then stir in the veg.