In a large saucepan, heat 1/4 cup water over medium heat. Add onion and cook until tender, about 3 minutes. Don't let the onion dry out.
Add remaining water, pumpkin, broth, cinnamon and nutmeg. Bring to a boil, reduce heat and simmer for 5 minutes. Stir in the milk and cook until hot. Don't boil.
Ladle soup into warmed bowls and garnish with black pepper and green onion tops. Serve immediately.