In a large pot over medium heat, melt butter. Add potato and onion and cook, stirring occasionally, until onion is translucent, about 8 minutes. Add chicken broth and bring to a boil. Cover, reduce heat to low, and cook until potato is tender, 10 to 12 minutes. Stir in pumpkin. Using an immersion blender, purée mixture until smooth. (Or, use a blender to purée mixture in batches, then return to pot.) Stir in 1/2 teaspoon salt, 1/8 teaspoon pepper, and nutmeg. Increase heat to medium-high and bring mixture to a boil. Cover, reduce heat to low, and cook for 10 minutes. Stir in cream and heat thoroughly. Season with salt and pepper, if desired. Serve hot.