Trim away the mushroom stems and use them for another purpose. Drop the mushroom caps into boiling salted water and simmer, covered, for exactly forty-five seconds. Drain immediately. Put the mushrooms in a glass jar.
Combine the remaining ingredients and blend well with a fork. Pour the marinade over the hot mushrooms, cover, and let stand in the refrigerator for two days. Use as a relish or as one of the dishes for antipasto or hors d’oeuvre.