1. Put a piece of cheese cloth or well rinsed out clean tea towel in a colander.
  2. Pour in the yoghurt and let it drain overnight in the fridge.
  3. Pour the water into a pan, add sugar, lemon juice, rhubarb and strawberries and simmer for 15 minutes.
  4. Stir the custard powder with 1 tablespoon of water into a porridge and add it to the rhubarb strawberry mixture in the pan while stirring.
  5. Keep stirring until a firm sauce is formed. Leave to cool.
  6. Beat the whipping cream until stiff and fold the drained yoghurt lightly into the whipped cream.
  7. Divide this mixture over 4 bowls.
  8. Spoon the cooled rhubarb strawberry sauce over the yoghurt and leave to set in the fridge for at least 1 hour.
  9. If necessary, garnish with a leaf of fresh mint.