Remove seeds and pulp from the pumpkin, peel the skin, and slice thinly. Mince the onion.

Heat butter in a pot over medium heat, add onion, and sauté until softened. Cover and steam over low heat for about 3 minutes. Add pumpkin and water, bring to a boil, then cover and simmer for 15 minutes until the pumpkin softens. Let it cool slightly.

Transfer the mixture to a blender and blend until smooth.

Return the mixture to the pot, add milk, and heat until just before boiling. Add salt to adjust the taste.