Finely grind the mustard seeds using a food processor or mortar and pestle. Puree the apple and all the other ingredients up to and including the salt, transfer to a pan along with the ground mustard seeds. Simmer for approx. 10 mins., stirring constantly. Mix in the mustard seeds.
Pour the hot mixture into the prepared jars, seal immediately, leave to cool. Leave to infuse in the fridge for several days to allow the mustard to release its full aroma.