Melt the butter then cook the onion gently in a covered saucepan for 5 minutes without colouring. Add the potato, 700 g of the pumpkin, the carrots and the stock.
Cover and bring to the boil, then simmer gently for about 20 minutes until the vegetables are tender. Cool a little, then puree in a liquidiser. Return to a clean saucepan and pour in the milk.
Meanwhile, add the remaining pumpkin to a saucepan of boiling salted water and cook for 2 minutes. Drain and add to the pureed soup.
Add the sugar, nutmeg and seasoning to taste then reheat gently to serve.