Simmer the potatoes in a large saucepan for 15-20 mins until just tender; drain.
Meanwhile, heat 1 tbsp oil in a flameproof casserole dish over a medium heat. Add the chicken and cook for 5 mins each side until golden and cooked through. Transfer to a plate, season with black pepper, cover and keep warm.

Reduce the heat to low and add 1 tsp oil and the onion to the dish. Fry for 3 mins or until the onion has just softened, scraping any bits from the base of the dish. Add a splash of water if needed to loosen. Add the garlic and fry for 1 min. Stir in the vinegar, capers and tomatoes. Cook for 5-6 mins until the tomatoes just start to burst, stirring occasionally. Return the chicken to the dish. Sprinkle with the parsley and serve with the potatoes.