Trim the mushrooms, peel and boil them in salted water with the juice of a lemon. As soon as the water comes to the boil, remove the mushrooms. Pick out all the small mushrooms. Chop these, adding the stalks, the chicken liver, etc., and the chopped onion, browned in butter. Mix all well, season, place a layer of this stuffing on each large mushroom, and bake in the oven, basting with butter, for 25 to 30 minutes, till the mushrooms are tender.