Tip the coconut into a food processor with the measurement of hot water. Process until the coconut is finely grated. Transfer to a bowl and leave to stand for 10 minutes.
Put the ginger, garlic, turmeric, sugar and lime juice in the food processor. Strain the coconut-infused water through a sieve and add the liquid to the processor, discarding the pulp. Process until smooth.
Scatter the pumpkin in a casserole dish. Add the chillies and pour in the coconut mixture from the processor. Cover with foil and place on the lower rack of the halogen oven.
Set the temperature to 200°C (392°F) and cook for about 30 minutes until the pumpkin is tender. While cooking, melt the butter in a frying pan on the hob and gently fry the onion for 8–10 minutes until very soft. Stir into the curry and cook for a further 5 minutes. Check the seasoning, adding salt if needed, and scatter with the pumpkin seeds. Serve with plain or flavoured warm naan bread and mango chutney.