Peel and chop the chestnut pumpkin and potatoes.
Finely chop the onion.
Melt butter in a pot and sauté the onion.
Add the chestnut pumpkin and potatoes and continue to sauté.
Pour in water or vegetable stock and boil until the ingredients are cooked.
Blend the cooked ingredients until smooth.
Add milk and bring to a boil again, seasoning to taste.
Serve warm.