Peel the marrow, remove seeds and cut into small cubes. Put into a bowl, sprinkling salt between the layers, and leave for 24 hours. Peel and chop the onions and cook in a very little water until tender. Add all the ingredients except the marrow and bring to the boil. Drain and wash the marrow and add to the pan. Simmer gently until the mixture is thick, about 2 hours. Pour into hot sterilized jars and seal. Leave for about 2 months before using.