Melt the butter in a saucepan and turn in the pumpkin and onion, stirring them for a few minutes on medium heat.
Add the sugar, pour in enough stock to surround but not drown the pumpkin, bring to a gentle simmer and cook, covered, for 15 minutes until the pumpkin is soft.
Mash the pumpkin with a potato masher or a blender and return to the pan. Add salt, pepper, and stock and bring it up to heat to simmer for a few minutes. Stir the soup and check the seasoning and consistency.
Serve with a neat drizzle of yoghurt, and a sprinkling of parsley.