Heat the oil in a large heavy-based saucepan or preserving pan and fry the black mustard seeds and fenugreek seeds until they start to pop. Stir in the ground ginger and onion and cook for 5 minutes until the onion is softened.

Add the rest of the ingredients and cook over a low heat until the sugar has dissolved, then bring to the boil. Reduce the heat again and simmer for 1–1½ hours until thickened.

Remove from the heat, then spoon the chutney into 6–8 sterilised 450g jars and seal. Set aside to cool completely. Leave to mature in a cool, dark place for a few weeks before eating. The chutney will keep for up to a year.