Melt butter in a Dutch oven over medium-high heat. Add onion; saute 3 minutes. Stir in flour, curry powder, cumin, nutmeg, and garlic; saute 1 minute. Add sweet potato, salt, broth, and pumpkin; bring to a boil. Reduce heat; simmer, partially covered, 20 minutes or until potato is tender, stirring occasionally. Remove from heat; cool 10 minutes.
Place half of pumpkin mixture in a blender or food processor; process until smooth. Pour pureed soup into a large bowl. Repeat procedure with remaining pumpkin mixture. Return soup to pan; stir in milk. Cook over medium heat 6 minutes or until thoroughly heated, stirring often (do not boil). Remove from heat; stir in juice. Garnish with chives, if desired.