How to Make It
Preheat the oven to 300°. Season the oxtails with salt. In a large enameled cast-iron casserole, heat the olive oil over moderately high heat. Working in 3 batches, brown the oxtails for 7 to 10 minutes, turning occasionally. Transfer to a large plate. Remove all but 2 tablespoons of fat from the casserole. Add the onions, tomatoes, celery, carrot and garlic and cook over moderately high heat until just starting to brown, about 15 minutes. Stir in the wine and sherry, bring to a boil and cook for 10 minutes. Return the oxtails to the casserole. Add 8 cups of water along with the bay leaf and peppercorns. Bring to a boil, then cover and transfer to the oven. Braise, stirring the oxtails a few times, until very tender, about 3 1/2 hours. Let cool, then cover and refrigerate overnight. Skim the fat from the top of the stew.
Gently reheat the stew over moderate heat. Transfer the oxtails to a large plate. Strain the sauce into a large bowl, pressing on the solids. Discard the solids. Return the sauce to the casserole and bring to a boil, then simmer for about 10 minutes, until slightly thickened. Season with salt. Return the oxtails to the sauce and simmer until warm. Transfer the oxtails and sauce to bowls, garnish with celery leaves and serve with bread.