Peel and finely slice the onions, then peel and grate the garlic and ginger. Peel, deseed and chop the pumpkin into 2cm chunks, then halve and deseed the chillies. Put aside for later.
Drizzle 2 tablespoons of oil into a large saucepan over a high heat. When the oil is smoking hot, add mustard and fenugreek seeds, then stir in the onion, garlic and ginger.
Reduce the heat to medium and cook, stirring, until the mixture is dark brown and smelling fantastic. Add the ground spices, cook for 1 further minute, then add the tomatoes, breaking them up with the back of a wooden spoon. Bring to the boil and simmer gently for a few minutes.
Finely grate the lime zest into the pan. Squeeze the lime juice into a measuring jug and top up to 500ml with cider vinegar. Pour into the pan, add the pumpkin and chillies, and bring back to the boil. Cook for a further 10 minutes, or until the pumpkin is just cooked.
Lift the pumpkin out with a slotted spoon and set aside. Add 1 ½ teaspoons of sea salt and the sugar to the pan, and continue cooking until the liquid has reduced and thickened.
Fold the pumpkin back into the pan, then spoon the pickle into sterilised jars. Seal the jars straight away and leave to cool. The pickle will be ready to eat in a few weeks time.