Remove the seeds from the pumpkin, peel it, and cut it into small pieces.
Thinly slice the onion.
Melt butter in a pot and sauté the onion.
Add the pumpkin and sauté together.
Pour in water and boil until the pumpkin is cooked through.
Transfer the contents to a blender and blend until smooth.
Return to the pot, add milk, and simmer while stirring.
Season with salt and pepper.