Heat the butter in a large saucepan. Slice the onion, then gently cook the onion for 5 minutes until soft but not caramelised. Peel, deseed and chop the kabocha pumpkin into bite size chunks. Slice the potato into 5mm thick rounds. Add the pumpkin and the sliced potato to the pan and continue to cook for 8-10 minutes, stirring occasionally until the potato and pumpkin are soft.
Pour the stock into the pan and bring to the boil. Turn the heat down and simmer until the vegetables become soft enough to smash with a fork. Then purée with a hand blender or a food processor. Put the soup back into the pan and add the milk and season with salt and pepper. Reheat the soup. Add more milk if needed to reach your preferred consistency.
Taste for seasoning, adjust the amount of salt and pepper. Serve with chopped parsley, croutons or pumpkin seeds. Enjoy warm.
- If you are unable to find kabocha in your market, you can try and substitute buttercup squash or pumpkin puree if those are available.