Bring the grape juice, sugar, lemon zest, except for something to garnish, and lemon juice to the boil. Stir in semolina. Bring to the boil again and let it swell over low heat for about 3 - 4 minutes. Cool a little, fill in 4 bowls and put in the fridge.
Garnish with the cream, the finely chopped walnuts, lemon zest and the halved grapes.