Lightly season fish fillet with salt and pepper.
Set aside. Saute onion in butter until transparent.
Stir in crab meat, button mushrooms and water.
Let simmer for a minute or two.
Add in fresh milk and season with salt and pepper.
Thicken with cornstarch. Heat oil for frying.
Slit an opening in the middle of the fish enough to fill at least 1 tablespoon of the crabmeat filling.
Fill fish with crab meat at least 1 tablespoon full.
Do not put too much filling so it won't spill out of the fish.
Cook fish on one side then carefully flip it over to cook the other side.
Before removing fish from the pan, place a slice of cheese on top of each fish and let it melt.
Remove from pan and serve hot with rice pilaf.