Put the juice, sugar, lemon peel and lemon juice in a small pot and bring to the boil over medium heat. Remove the pot from the heat and add the semolina while stirring. Bring to the boil again and simmer for 3-4 minutes at low heat, stirring constantly. Let it cool down a little bit, fill it into bowls and let it set in the fridge. Coarsely chop the walnuts.
Wash, halve and seed the grapes. Spread on the grape semolina. Put a blob of yoghurt on each dessert and sprinkle with walnuts and possibly lemon peel strips. Serve decorated with lemon balm as desired