Bercy is the simplest of the white-wine fish sauces. The poaching liquid is thickened with
beurre manie a flour and butter paste then enriched with cream. This combination of fish, mushrooms, and cream sauce is an informal version of sole bonne femme. Serve with it a white Burgundy, Graves, or Traminer.
3/4 lb. or 3-1/2 cups sliced fresh mushrooms
2 Tbsp butter
1/8 tsp salt
Pinch of pepper
2-1/2 lbs. filets of sole or flounder
1-1/4 to 1-1/2 cups cold, white wine fish stock made from heads, bones, and trimmings
OR 1/4 cup dry white wine or 2/3 cup dry white vermouth plus 1/4 cup bottled clam juice, and water
OR 1-1/2 cups wine and water mixed
Buttered brown paper or waxed paper (do not use aluminum foil-it will discolor the wine)
2-1/2 Tb flour blended to a paste with 3 Tb softened butter
3/4 to 1 cup whipping cream
Salt and pepper
1/4 cup grated Swiss cheese
1 Tb butter cut into bits
Preheat the oven to 350°F (175°C).
In an enameled skillet, toss the mushrooms in hot butter over moderately high heat for a minute or two without browning. Season with salt and pepper, and set aside.
Season the filets lightly with salt and pepper and arrange them in one slightly overlapping layer in the dish. If filets are thin, they may be folded in half so they make triangles. Spread the mushrooms over them.
Pour in the cold liquid and enough water so fish is barely covered. Bring almost to the simmer on top of the stove. Lay the buttered paper over the fish. Then place dish in bottom third of preheated oven. Maintain liquid almost at the simmer for 8 to 12 minutes depending on the thickness of the filets. The fish is done when a fork pierces the flesh easily. Do not overcook; the fish should not be dry and flaky. Place a cover over the dish and drain out all the cooking liquid into an enameled saucepan.
Rapidly boil down the poaching liquid until it has reduced to 1 cup.
Off heat, beat the flour and butter paste into the hot liquid, then 1/2 cup of the cream. Bring to the boil. Thin out the sauce with additional tablespoons of cream until it coats the spoon nicely. Season to taste with salt, pepper, and drops of lemon juice.
Spoon the sauce over the fish. Sprinkle on the cheese, and dot with butter. Place dish 6 to 7 inches from a hot broiler for 2 to 3 minutes to reheat fish and brown top of sauce lightly. Serve as soon as possible.
(*) Dish may be prepared ahead and reheated as follows: After sprinkling on the cheese and butter, set aside. Before serving, reheat just to the simmer on top of the stove, then run for a minute or two under a hot broiler to brown the top of the sauce.