Remove the seeds from the sweet pumpkin, peel it, and cut into appropriate sized pieces.
Thinly slice the onion.
Melt butter in a pot and sauté the onion.
Add the sweet pumpkin and cook together.
Pour in water (or broth) and boil until the sweet pumpkin is tender.
Blend until smooth with a mixer.
Transfer back to the pot, add milk and salt, and heat while stirring.