Melt butter in a Dutch oven or large saucepan over medium-high heat. Saute onion for 3-4 minutes then add flour, curry, cumin and nutmeg and saute for 1 minute.
Add sweet potato, salt, chicken broth and pumpkin and bring to a boil. Reduce heat to medium-low and simmer, partially covered for about 20-25 minutes or until sweet potatoes are cooked through and softened. Remove from heat and let stand for 10 minutes to cool.
Place half of the pumpkin mixture in a blender and process until smooth. Using a strainer, pour soup back into pan. Repeat with rest of soup.
Raise heat to medium then stir in milk and cook for 5 minutes or until soup is heated through.