Cut butternut squash in half length wise. Remove seeds and fibres and brush cut side with olive oil. Place with the cut side down on a baking tray.
Roast in the preheated oven till the skin is wrinkled and the squash is soft, about 40 minutes.
Let cool slightly, then scoop out the flesh with a spoon and set to one side.
Heat butter in a large pot and fry onion till translucent.
Add squash, ginger and 500ml hot stock. You can add the ginger minced and puree it with the soup later, or, for a less intense ginger flavor, add the ginger sliced, and remove it before pureeing the soup. Add curry, cumin, salt, pepper and sugar.
Bring to the boil, then reduce the heat and simmer for 15 minutes. Puree the soup in a blender or with an immersion blender.
Return soup to the pot and add more stock as liked to reach the desired consistency. Stir in cream and just the seasoning with salt, cayenne pepper, sugar and a dash of vinegar.