Slice the top off the pumpkin and carefully remove all the seeds. Using a knife or a heavy spoon remove the flesh from the insides. Be careful not to cut through the shell of the pumpkin if you are using it as a serving dish, or the soup will leak out. Chop the pumpkin flesh quite coarsely.
Heat the oil and butter in a large heavy-based pan. When it is foaming add the onion, ginger and chillies and cook for 5-6 minutes, stirring occasionally. Next add the pumpkin flesh and stir well. Place the lid on the pan and leave to simmer for 8-10 minutes. Give the mixture a stir from time to time to prevent it sticking.
Add the tomatoes and cook for 4-5 minutes. Now add the chicken stock and season with salt and pepper. Replace the lid and cook for a further 45 minutes.
Remove the soup from the heat and allow it to cool slightly. Pour the soup into a blender or liquidiser and blitz until it reaches the consistency you prefer.
Return the soup to the saucepan, add the coconut milk, adjust the seasoning and cook for a further 6-7 minutes.
To prepare the pumpkin for use as a serving dish, ensure it is well scooped out and dry. Place in a pre-heated oven @ 180°C/gas mark 4 for 10 minutes. Just to warm it through. Now add the soup. Garnish with the herbs and slices of chilli.