Place chicken and mushrooms into a large shallow dish. Combine oil, lemon juice, thyme, salt and pepper in a jug. Whisk well to combine. Pour over chicken and mushrooms. Toss well to coat.
Preheat a greased barbecue plate or large frying pan on medium-high heat. Add chicken and cook for 4 - 5 minutes on each side or until cooked through. Remove to a plate, cover with foil and keep warm. Increase heat to high, add mushrooms and cook, tossing frequently, for 2 - 3 minutes or until tender. Place chicken onto serving plates, top with mushrooms and season with salt and pepper.