Combine the balsamic, Worcestershire, mustard and rosemary or thyme in a jug. Pour half the mixture into a shallow glass or ceramic dish. Add the chicken and turn to coat. Cover and place in the fridge for at least 30 minutes to marinate.
Meanwhile, combine the stock, cornflour and remaining balsamic mixture in a small jug.
Heat 2 tsp oil in a large non-stick frying pan over medium-high heat. Cook the chicken (discarding excess marinade) for 3-4 minutes each side or until golden and almost cooked through. Transfer to a plate.
Reduce heat to medium. Heat the remaining oil in the pan. Add the leek and cook, stirring, for 5 minutes or until softened. Add all the mushrooms and garlic. Cook, stirring, for 5 minute or until mushrooms are golden. Return the chicken to the pan. Pour in the stock mixture and simmer for 3-4 minutes or until the chicken is cooked through and mushrooms are tender. Sprinkle with parsley. Serve with green beans.
