Step 1Place oxtails and butter in Dutch oven or large soup pot. Add carrots, turnips, onions, leek, celery, herbs, bay leaf, peppercorns, cloves, ham and 1 cup water. Cook over high heat, stirring occasionally, until vegetables soften, 8 to 10 minutes.
Step 2Add remaining water, except for 1/2 cup, and bring to boil. Add salt. Skim fat and scum. Simmer gently until oxtails are tender, 3 to 3 hours 30 minutes.
Step 3Remove oxtails and set aside. Skim fat and scum again. Strain soup, discarding vegetables, herbs, spices and ham, and return soup to pot.
Step 4Combine flour and 1/2 cup water and add to soup to thicken. Add ketchup and Port. Return oxtails to soup and simmer 5 minutes.