Roughly chop the tomatoes, stir in the salt and leave to steep for about an hour. Meanwhile measure and prepare all the other ingredients to make it easier to cook.
Warm the vinegar on a low heat in a small saucepan. Add the mustard seeds then remove from the heat and allow to infuse for 15 minutes.
Put the garlic and ginger into a blender and pulse a few times. Add the vinegar and mustard seed infusion and blend into a smooth paste.
Heat the oil in a large heavy bottomed saucepan on a high heat for one minute. Take off the heat for a moment then add the crushed coriander seeds, cumin seeds, turmeric, nigella seeds, cloves and chilli powder.
Fry on a moderate heat for 20 seconds then add the mustard vinegar paste and the chillies and onions. Fry for a further 5-10 minutes.
Drain the excess liquid from the tomatoes and add the tomatoes to the pan with the apples and sugar. Reduce the heat and simmer gently for an hour, stirring occasionally.
Ladle into warm sterilised jars and seal. It can be eaten straight away but is better if left in a cool dark place for 4 weeks. It will keep unopened for up to a year. Once opened, refrigerate and eat within 4 months- though its unlikely to last that long….