Arrange the fish pieces on a large lightly greased
heatproof plate. Scatter over the mushrooms and set
aside.
Combine the cornstarch and soy sauce until they are
well blended, then gradually stir in al of the
remaining ingredients. Pour the mixture over the
fish. Cover with foil or a second heatproof plate,
and arrange in the top half of a stemmer.
Half-fill the bottom half of the steamer with
boiling water, set the pan over moderately low heat
and steam the fish for 10 to 15 minutes or until the
flesh flakes easily.
Remove from the heat and transfer the fish and sauce
mixture to a warmed serving dish. Serve at once.