Mix the blueberries and raspberries with the sweetener, lemon juice, orange juice, and lemon zest in a heat-resistant bowl.
Cover with plastic wrap.
Freeze the 4 tablespoons of yoghurt.
Put the covered bowl in a large pot with boiling water over low heat and cook for 10 minutes. Set aside until it cools off. Refrigerate until cold, around 4 hours.
Divide the fruits and their liquid into 4 plates, decorate with fresh mint, and pour a spoonful of frozen yoghurt onto each plate.