Photograph: Karen Thomas
Put the potatoes in a pan of cold salted water, bring to the boil, then simmer for 10-12 minutes or until tender. Drain, allow to cool a little, then peel the skins from the potatoes (or you can leave them on if you prefer). Slice lengthways into three and return to the pan.
For the dressing, put the grapes in a food processor and blitz for 1 minute on high to purée, then pass through a sieve set over a bowl to press out the juice. Discard the grape skins. Whisk the oil and lemon juice into the grape juice and season. Drizzle a little of the grape dressing over the potatoes and season lightly.
Cut each chicken breast fillet lengthways into 5-6 wide strips. Put the mustard in a large shallow bowl, add the chicken and season with salt and pepper. Sprinkle with the flour and toss to coat.
Pour in enough olive oil to just cover the base of a large nonstick frying pan. Set over a moderate heat, add a third of the chicken strips and fry for 6-7 minutes, turning once. When golden and cooked through, transfer to a tray lined with kitchen paper to drain. Repeat with the remaining two batches of chicken strips.
On a platter, arrange the potatoes and chicken strips with the rocket and capers. Drizzle over the remaining grape dressing and serve.
Get ahead: make the dressing and cut the chicken into pieces the day before; chill. Cook, peel and slice the potatoes a few hours ahead. Cook the chicken up to an hour ahead.
Kitchen secret: piccata is traditionally made with veal but chicken is the top choice in the US.