Layer the potato slices and thyme in a well-greased, ovenproof dish. Mix together the oil and stock and pour over the potatoes. Dot over the butter, cover with foil and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 1 hour, removing the foil halfway through cooking.
Meanwhile, put the chicken in a non-metallic dish. Mix together the lemon juice, garlic and tarragon, pour the mixture over the chicken and leave to marinate for 30 minutes.
Heat the butter in a frying pan and fry the mushrooms, then add the chicken and any juices and fry for a further 3 minutes. Add the cream to the pan and season with salt and pepper. Simmer gently for a couple of minutes until the chicken is just cooked. Serve with the potatoes and steamed green beans.