Preheat oven to 120˚C / 100˚C fan-forced. Halve chicken fillets horizontally. Cover each piece in plastic wrap and using a meat mallet or rolling pin, flatten until chicken is about 1/2 cm-thick. Place flour onto a large plate. Season. Lightly dust chicken in flour.
Heat oil in a large frying pan over a medium-high heat. Cook chicken, in 2 batches, for 2-3 minutes on each side or until just cooked through. Transfer to a baking tray. Cover with foil and keep warm in the oven.
Add butter, mushrooms and garlic to the pan. Cook, stirring often, for 3-4 minutes or until mushrooms are just tender. Stir in stock and bring to the boil. Cook for 2 minutes. Stir in lemon juice. Season.
Arrange chicken on a serving platter. Spoon mushroom mixture over chicken. Sprinkle with parsley. Serve with steamed green beans and lemon wedges.