Combine sweet potato, sliced leek, and 1/4 cup broth in a 1 1/2-quart casserole; stir well. Cover, and microwave at HIGH 10 minutes, stirring after 5 minutes. Place sweet potato mixture in a blender or food processor; process until smooth. Add remaining ingredients except chopped leek; process 30 seconds or until blended. Garnish with chopped leek, if desired. Serve warm.