Put the black mustard seeds into a non-metallic bowl and pour over 150 ml of the vinegar. Cover and leave overnight.
The next day, use a mortar and pstle to pound the mixture until the seeds are coarsely broken. Grind the yellow mustard seeds in an electric blender or coffee grinder to a very fine powder. Combine the two mixtures and stir in the rest of the vinegar and the salt.
Spoon into sterilized jars. Seal, label and keep in a cool dark place for 2 weeks before using to allow the flavours to develop.