Melt butter in a 6-quart pressure cooker over medium heat. Add onion; cook 5 minutes or until tender, stirring occasionally. Add the potato and broth. Close lid securely; bring to high pressure over high heat. Adjust heat to medium or level needed to maintain high pressure; cook 5 minutes. Remove from heat; place cooker under cold running water. Remove lid; let soup cool for 10 minutes.
Place half of soup in a blender or food processor; process until smooth. Pour pureed soup into a large bowl. Repeat procedure with remaining soup. Stir in salt, pepper, and nutmeg. Add milk and sour cream, stirring until well blended. Cover and chill. Top with chives just before serving.