1. Wipe fresh mushrooms with a damp cloth. (Cut off and discard stem ends.) Place in a bowl.
2. Mince onions. Combine in a saucepan with sugar, soy sauce, sherry, vinegar and salt. Bring to a boil, stirring. Then pour over mushrooms and toss gently.
3. Refrigerate, covered, 24 hours. Drain and serve.