Heat the oil in a flameproof casserole large enough to hold the chicken breasts in a single layer. Add the chicken and brown them all over.
Season the chicken with salt and pepper, then add the lemon juice, mushrooms and stock. Cover and bring to the boil, then lower the heat and simmer, stirring occasionally, for 20 minutes, or until the chicken juices run clear when the breasts are pierced with a knife. Add a little more stock or water if the liquid is evaporating too quickly; there should be some liquid left when the chicken is cooked.
Remove the chicken breasts from the casserole and keep them warm. Boil the remaining cooking liquid until it thickens slightly. Stir in the chopped thyme or marjoram and spring onions and adjust the seasoning. Serve the chicken, spoon the mushroom sauce over it and garnish with sprigs of thyme.