Wash fish, cover inside and out with salt; let stand 2 to 3 hours.
Wash and pat dry, sprinkle inside lightly with salt.
Shape pan of heavy foil closely around the fish; set in larger pan.
In a small pan melt butter; saute mushrooms and onions about 5 minutes.
Stir in the soy sauce, ginger and salt; then stir in the cornstarch blended with sherry.
Cook until thickened and then pour over fish.
Tightly cover the pan and fish with tent of heavy foil.
Bake in a 400° oven for 7 to 10 minutes per pound.