Sprinkle lemon or lime juice over pumpkin and leave to stand for 20 minutes. Shell and devein fresh prawns or wash dried shrimp.
In a food processor, blend together prawns, onions, shrimp paste, chillies, lemon grass and a little water until they form a smooth paste.
Mix together water and thin coconut milk in a large saucepan. Add the paste, fish sauce, sugar and pepper, whisking to combine thoroughly. Bring mixture to a boil, then reduce heat and add pumpkin. Simmer over low heat until pumpkin is tender, about 20 minutes. Adjust seasoning if necessary.
Just before serving, stir in basil leaves and thick coconut cream. Garnish with a few basil leaves.