Coat the oxtails with the flour. Melt the butter in a large soup pot or dutch oven. Add the meat and brown thoroughly. Add the chopped vegetables and continue to cook until the vegetables are slightly softened.
Add the peppercorns, herbs, salt, stock, and water. Bring to a boil, and skim off any scrum that forms on the surface.
Reduce to a simmer, cover, and cook over low heat for 4 hours or until the meat is completely tender and falling off the bones.
Carefully strain the solids out of the stock, reserving the stock and putting it back in the soup pot. Let the meat cool until you can handle it, then strip the meat from the bones. Discard the bones and fatty skin, and place the meat back in the soup.
Puree the vegetables in a blender of food processor, and return to the soup.
Reheat the soup to serving temperature, then add the sherry. Taste, and add salt or pepper as needed before serving.