Oxtail is traditionally cooked in red wine, but Guinness stout makes a good substitute.
Adapted from “The Country Cooking of Ireland” by Colman Andrews.
Preheat oven to 350 degrees.
Melt butter with the oil over medium-high heat in a Dutch oven or flameproof casserole with a lid. Brown the oxtails on both sides, working in batches if necessary. As oxtails brown, remove from Dutch oven and set aside.
Reduce heat to medium and put onions and carrots into the Dutch oven, adding more oil if necessary. Cook, stirring frequently, for 10-12 minutes or until onions and carrots soften and begin the brown. Sprinkle flour over them and stir in well, then add Guinness and tomatoes and enough water to just cover all the ingredients, stirring everything together well.
Add the bay leaves to oxtails and season to taste with salt and pepper, then cover the Dutch oven tightly and cook for 4 hours, stirring contents about every 45 minutes.
Remove oxtails from the liquid and spread out on a baking sheet to cool. When cool enough to handle, pull meat from bones with your fingers or a knife and fork, saving bones for stock. (If using your fingers, be very careful: bones can be sharp.)
Return the meat to Dutch oven or casserole dish and mix it in well. Cook over medium heat for 5-6 minutes to heat through. Liquid should be thick and dark. If it’s still too thin, simmer, uncovered, over low heat, stirring frequently, for 20–30 minutes, or until it thickens.