Combine first 3 ingredients in a deep 3-quart casserole.
Cover with heavy-duty plastic wrap, and microwave at HIGH 14 to 16 minutes or until vegetables are tender.
Pour mixture into container of an electric blender or food processor; top with cover, and process until smooth.
Transfer to a medium bowl.
Add milk, salt, pepper, and nutmeg; stir well.
Cover and chill thoroughly.
To serve, ladle soup into individual bowls.
Garnish with chopped fresh chives, if desired.