Ask the fish dealer to give you the heads, tails, and skin from the flounder.
Place these trimmings and small chunks of the halibut or cod in a saucepan.
Toss in carrots, leek, parsley, peppercorns, bay leaf, and thyme.
Add water and cook to a boil, then reduce heat, and simmer gently until liquid is reduced to about 1 pint.
Strain through a fine cheesecloth, saving the stock.
Arrange fillets in a buttered baking dish and sprinkle with salt and pepper.
Add 1 cup of the fish stock and bake in a preheated 325° oven for 15 to 20 minutes.
With a broad spatula, carefully transfer fillets to a hot ovenproof serving platter.
Arrange oysters and shrimp on top.
Set aside.
Pour remaining cup of fish stock into pan in which fillets baked.
Cook until stock is reduced to about 1/4 cup.
Strain into top of double boiler, add white wine and butter.
Cook over hot water, stirring until butter is melted.
Beat egg yolks vigorously, then stir in the butter mixture a little at a time.
Pour egg mixture into top of double boiler and cook, stirring constantly, until sauce is the consistency of a medium cream sauce.
Pour over fish.
Broil in a preheated broiler until golden brown.