French Pumpkin Soup

Melt the butter in a large heavy saucepan over medium-high heat. Add the onion and cook, stirring, until it begins to turn translucent, about 5 minutes. Add the pumpkin, the potato, and the water; stir, and then season lightly with salt. Cover and bring to a boil. Reduce the heat to medium and cook, covered, until the pumpkin is so soft that it comes apart, 25 minutes. Remove from the heat and purée the mixture in a food processor or with a wand mixer until smooth and uniformly blended. Taste for seasoning, and serve with the cheese alongside.