Open the mussels by putting them in a large pan over a brisk fire, with a damp tea towel on top of them.
After 5 minutes the shells will open; keep the mussels, throw away the shells.
Strain the liquid which will have come out of them, and keep.
Peel the shrimps, and put the shells into the mussel liquid.
Open the oysters.
Pour the water from them into a pan and poach them in it gently, with the addition of a little white wine, for 10 minutes.
Skin the soles, fillet them, and keep the fillets and the skins and the bones.
Put into a pan together the liquid from the mussels, the shells of the shrimps, the water in which you cooked the oysters, the heads, skin and bones of the soles, a bunch of mixed herbs, and enough white wine to ensure that when, later on, you put the fish in they will be covered over.
Put the lid on the pan and simmer the strong stock for 1 hour.
Then strain through muslin or a fine sieve or napkin.
Poach the fillets of sole in the carefully strained stock: cook on a gentle heat for 8 to 10 minutes.
Now take them out and drain dry.
Make a pale roux by melting 2 tablespoonfuls of butter and stirring in 1 tablespoonful of flour; mix this very gradually with the stock, adding the mushrooms, sliced but not peeled.
Simmer for 10 minutes, then away from the heat bind this sauce with the yolks of egg beaten up with lemon juice.
Arrange the soles on a fireproof dish, surround them with mussels, oysters and shrimps, and pour the sauce over them, heating in a hot oven for a few minutes to make the surface of the sauce glisten.
Serve very hot.
Note: This dish, which is exquisite, yet quite easy to do, can be garnished with pieces of bread fried in butter, shrimp-tails, fried smelt or gudgeon, or slices of truffle—or all these at once.
The oysters may of course be omitted, if desired.
The recipe can be used equally well for fillets of plaice.