1. Combine first 4 ingredients in a medium saucepan; bring to a boil. Reduce heat, and simmer for 20 minutes or until rhubarb is tender. Cool slightly. Place rhubarb mixture in a blender, and process until smooth. Pour into a bowl, and refrigerate for 2 hours or until thoroughly chilled. Pour mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Place a freezer-safe bowl in a freezer for 10 minutes. Spoon sorbet into bowl; cover and freeze for 4 hours or until firm. Garnish with mint and rind, if desired.