Put the mushrooms in a heatproof bowl, and cover with boiling water. Let stand for 15 minutes. Reserve half a cup of the soaking liquid, drain the mushrooms. Remove the stems from the mushrooms.
Combine the mushrooms, mushroom liquid, soy sauce, sherry vinegar, sugar in a saucepan. Bring it to a boil, reduce it to a simmer, and cook for 30 minutes.
Remove the pan from the heat and allow to cool down to room temperature.
