Wash the sweet pumpkin thoroughly, remove the seeds, and cut into bite-sized pieces.
Thinly slice the onion.
Melt butter in a pot and sauté the onions.
Add the sweet pumpkin and sauté together.
Pour in water or stock and boil until the pumpkin is cooked.
Blend the cooked ingredients until smooth.
Transfer back to the pot, add milk and salt, and bring to a boil again.