Combine first 4 ingredients in container of an electric blender; cover and process until smooth, stopping once to scrape down sides. Spoon mixture into a bowl; cover and freeze until firm.
Remove orange mixture from freezer 1 hour before serving; let thaw partially. To serve, break orange mixture into chunks, add mineral water, and stir until slushy. Garnish with orange slices, if desired.
Cooking Light Light Cooking for Two