Pull the grapes off the stalks and reserve 200 g (7 oz). Put the remainder in a saucepan with 3 tablespoons of water. Cover and simmer over a low heat for 20 minutes, or until they soften and burst. Crush them thoroughly to release their juice, then cover and simmer for a further 10 minutes. Strain and reserve 350 ml (12 fl oz) of the liquid, then set it aside.
Meanwhile, cover the reserved grapes with boiling water and leave for 15 seconds. Drain, peel and halve them and remove their pips. Toss them in half the lemon juice.
Cut the chicken breasts into 2.5 cm (1 in) pieces and season. Heat the oil in a saucepan over a medium heat. Add half the chicken and fry briskly until coloured. Remove, repeat with the remaining chicken and set both batches aside.
Reduce the heat, add the garlic and shallots to the pan and sauté for 3 minutes, or until soft. Sprinkle the flour in and cook, stirring, for a further 1 minute. Stir in the lemon zest, the remaining lemon juice and the grape liquid, then bring to the boil and cook for 2 minutes.
Reduce the heat and return the chicken, with its juices, to the pan. Season to taste and simmer for 7 minutes, or until the chicken is tender and the juices run clear when the breasts are pierced with a knife at their thickest point.
Add the halved grapes, their juice and the chopped parsley and adjust the seasoning. Serve hot, garnished with sprigs of parsley.