Heat orange zest, juices, sugar and ginger and simmer until a thin syrup consistency. Remove from heat and cool. In a blender, purée canteloupe with cooled syrup until smooth. Add yoghurt, sour cream and vanilla and purée until smooth. Chill for at least 4 hours before serving.
Simply freeze soup in a non-reactive container, stirring once after the first two hours, until completely frozen. To serve, scrape up granules with a fork and spoon into tall glasses, garnish with a sprig of fresh mint.