Heat a large pot on a medium heat. Add the butter and brown the oxtail pieces on all sides.
Remove meat from the pot. Add the onions, carrots and celery. Cook for about 10-15 minutes until they have softened slightly. Make sure that you watch the pan and scrape off any meat that has stuck to the bottom
Add the garlic, tomato purée, salt, pepper, thyme and bay leaves
Pour in the red wine if you’re using it and let it cook for 2-3 minutes
Add the oxtail back to the pot and pour in the beef stock. Bring to a simmer and then reduce to a low heat to cook for 2 ½ hours
Once the meat is soft and tender you may want to take out the oxtail and remove the meat from the bone. It should fall off easily and then you can add it back in. Otherwise you can just leave it whole
Take out the bay leaves and serve