Wash the sweet pumpkin thoroughly, remove the seeds, and cut into bite-sized pieces.
Dice the onion and potato as well.
Melt butter in a pot and sauté the onion.
Add the potato and sweet pumpkin and stir-fry briefly.
Pour in water (or broth) and cook until the ingredients are tender.
Blend the cooked ingredients until smooth.
Return the blended mixture to the pot, add milk, and simmer until thickened.
Season with salt and pepper to taste.