Finely chop the onion, garlic, ginger and chilli in a blender or food processor, or finely chop by hand, and mix with the crushed cumin, coriander and cardamom.
Slice the pumpkin into 2.5 cm (1 inch) wedges, then cut the wedges in half. Heat the oil and butter in a large frying pan, add the pumpkin pieces and fry for 5 minutes until lightly browned. Push the pumpkin to one side of the pan then add the onion mixture and fry until beginning to colour.
Add the turmeric, paprika and cinnamon, cook briefly then stir in the stock. Season and bring to the boil. Cover and simmer for 10 minutes until the pumpkin is almost cooked. Allow to cool, cover and chill until required.
When ready to serve, add half the cream, half the pistachios and half the coriander leaves. Reheat until piping hot. Drizzle with the remaining cream, and sprinkle with the remaining pistachios and coriander. Serve with naan breads and a tomato and onion salad.