In a saucepan, combine the pumpkin, stock or water, ginger, and lemongrass. Cover, bring to a boil, reduce the heat to medium-low, and simmer until the pumpkin is very tender, about 12 minutes. Add the scallions and cook briefly.
Transfer the contents of the saucepan to a blender or food processor and process until the soup is partially puréed. Pour in half of the coconut cream and process until smooth.
Rinse the saucepan and pour the purée into it. Add half of the remaining coconut cream. Season with salt and pepper and heat through without allowing the soup to boil. Taste and adjust the seasoning and squeeze in lime or lemon juice.
If using the kaffir lime leaves, fold them in half and, using a sharp knife, trim away the hard central rib. Cut the leaves into threadlike shreds. Ladle the soup into bowls, add the remaining coconut cream, forming a swirl on top of each bowl, and garnish with the lime leaf or zest.