In a large bowl combine grated courgettes with half the salt and let rest for 1 hour. Rinse with cold water and drain well. Place into a large saucepan.
Toast mustard seeds, turmeric and coriander in a small frying pan until mustard seeds pop and spices are fragrant; add to the courgettes. Stir in apple cider vinegar, black pepper and whole dried chilli; bring to the boil and simmer for 25 minutes.
Add sugar and remaining salt. Continue to simmer for 1 hour, or until most of the liquid has evaporated and the chutney has thickened.
Fill very hot chutney into sterilised jam jars and screw lids on tight. Turn jars upside down and leave to cool. Store in a cool, dark cupboard.