Preheat the oven to 200°C. Chop the trimmed rhubarb stalks into 5cm lengths and toss through the brown sugar and cinnamon in a baking dish. Squeeze half a lemon over the rhubarb and cover the dish with foil and bake for 30 mins.
Allow to cool slightly before pureeing the rhubarb. Then lightly fold in the custard and the mixed berry yoghurt and pour into glass bowls and chill in the refrigerator for 1 hour before serving.