Wash the mushrooms carefully, dry thoroughly and peel them, removing the stalks. Boil in salted water till tender, and drain on a sieve. When quite cold, pack them in a jar and cover with the vinegar, which should have been previously simmered for 10 minutes with the peppercorns and salt, and allowed to stand till cold. Lastly, add the oil. Cover the jar with paper or a tight-fitting lid, and stand in a cold place for a few days.
Vinegar should never be put in an aluminium saucepan, as it is apt to dissolve the aluminium.