Wash the pumpkin thoroughly, remove the seeds, and cut into bite-sized pieces.
Thinly slice the onion.
Melt butter in a pot and sauté the onion.
Add the pumpkin and sauté together.
Pour in water or milk and simmer until the pumpkin becomes tender.
Blend the cooked ingredients in a blender until smooth.
Return the blended soup to the pot, season with salt and pepper.
Bring to a boil one more time, then serve in a bowl.