Oxtail may be bought skinless or skin-on. Skinless oxtail will cook in less time because the skin requires a long simmering time to soften. However, skin-on oxtail yields a richer stew. The skin turns gelatinous and acquires the mouth feel of beef tendon when sufficiently cooked. It's up to you whether to use skin-on or skinless oxtail.
Rinse the oxtail and dry on a stack of paper towels.
Sprinkle both sides with salt, pepper and flour.
Heat the olive oil in a wide thick-bottomed pan and brown the oxtail on all sides.
Scoop out the oxtail and move to a plate.
Make the stew base
In the remaining oil, saute the onion, garlic, bay leaves and cloves. Cook, scraping the bottom of the pan to loosen browned bits, until softened.
Add the bell peppers, carrot and leeks, and cook, stirring occasionally, for two to three minutes.
Pour in the wine.
Add the diced tomatoes and broth.
Stir and heat until bubbly.
Stew the oxtail
Add the browned oxtail to the sauce and bring to the boil.
Lower the heat, cover the pan and simmer the oxtail until tender, about four hours. Add more broth, half a cup at a time, if the liquid dries out during cooking.
The stew can be served when the oxtail is tender. But, if you want to add better texture and extra flavor, do the following.
Finishing touches
Separate the oxtail from the sauce.
Using an immersion blender, puree the sauce with the vegetables to make a smooth gravy.
Pour the sauce into a clean pan, add the oxtail, mushrooms and onion, and simmer for fifteen minutes or so until the mushrooms are cooked through.
Give the stew a taste and add more salt if needed (not likely if you used well seasoned beef broth) before serving.