Heat the oil in a large preserving pan, then gently fry the onions, ginger, chilli, cardamom, cinnamon, mustard and cumin seeds together for 5 mins, until the spices are aromatic.
Stir the garlic, squash and apples into the onions, then cook for 10-15 mins more, until the onions and apples are soft and the squash yields a little here and there.
Stir in the turmeric and sugar and let it melt around the vegetables. Simmer for 5 mins – this process almost candies the chunks of pumpkin, so that it doesn’t entirely break down during the next step.
Pour in the vinegar, season with 2 tsp salt, then bring the chutney back to a simmer. Cook, stirring regularly, for about 30 mins or until the apple has cooked down to make a squishy base for the chutney, with chunks of tender pumpkin here and there, and a little syrupiness at the bottom of the pan – you don’t want the chutney to be too dry as it will thicken as it cools.
Spoon the hot chutney into sterilised jars and seal. The chutney can be eaten straight away, or left to mellow in a dark place. You can store it for up to six months.