Wash the sweet pumpkin thoroughly, remove the skin and seeds, then cut into small pieces.
Chop the onion and potato finely.
Melt butter in a pan and sauté the onion.
Add the potato and sweet pumpkin, and stir-fry together.
When the vegetables become tender, pour in water and bring to a boil.
Blend the cooked vegetables until smooth in a blender.
Transfer the puréed vegetables back to the pot, add milk, and simmer.
Season with salt and pepper to taste.