Put the balsamic vinegar in a bowl and season with pepper. Make 3 small cuts in the top of each of the chicken breasts. Add the chicken to the vinegar and toss to coat. Set aside for 3–4 minutes.
Meanwhile, heat the oil in a large, heavy-based frying pan and cook the onions over a moderately high heat for 5 minutes or until soft and beginning to turn golden. Add the chicken, cut side down, and cook for 3 minutes. Turn and cook for a further 3 minutes.
Turn once more and add the balsamic vinegar from the bowl together with the honey and rosemary. Reduce the heat, add the stock, cover and simmer, stirring once, for 3–4 further minutes or until the chicken is cooked through. Serve the chicken on warmed serving plates with the onions spooned over. Serve with mashed potatoes and green beans.