Chop the potato into 3 cm chunks. Trim the green beans and slice in half. Butterfly chicken breast. Finely slice the leek.
Place the potato in a large saucepan of cold salted water. Bring to the boil and cook for 15 minutes or until soft. Add the green beans in the last 2 minutes of cooking time. Drain and return to the saucepan. Toss with butter and rosemary. Season with salt and pepper. Set aside.
Lay the chicken breast on an oven tray. Rub with olive oil and season. Place under the grill for 10 minutes, turning half way, until the chicken is just cooked through. Remove from under the grill.
While the chicken is grilling, heat a drizzle of oil in a frying pan. Add the leek and cook for 8 minutes until soft and sweet. Remove from the heat. Crumble in the fetta cheese and season to taste. Top chicken with leek mixture and chilli flakes and return to the grill for 1-2 minutes until crispy. Serve with lemon wedges.