Top and tail the gooseberries. Mix the sugar with half the vinegar and boil until a syrup forms. Add the chopped onions, raisins, bruised mustard seeds, spice and salt. Boil the gooseberries in the remaining vinegar until tender. Put the two mixtures together, and simmer for 1 hour until golden brown and thick, stirring occasionally to prevent sticking. Put into hot jars, cover and seal.