A day or two ahead -- In saucepan combine all ingredients except whole oranges. Over medium heat, gently boil for 10 minutes, stirring occasionally, until the cranberries have popped and the relish is red and beginning to thicken. Let cool and put in serving bowl. Chill thoroughly, covered in refrigerator. Remove relish from refrigerator a couple of hours before serving so it is about room temperature for serving. Meanwhile cut top 1/3 off each orange using a zig-zag cut. Hollow out the pulp with a spoon taking care not to pierce the shell. Reserve pulp and juice for another use (see note below). Orange cups will keep in plastic bag in refrigerator up to two hours. When ready to serve, place orange cups in a small custard dish or ramekin, one for each person, and fill 3/4 full with room temperature cranberry relish. Garnish with mint sprig and serve.