In a large wide saucepan bring the fish stock, wine, shallots, celery, parsley, bay leaf, peppercorns and salt to a simmer and cook for 20 minutes covered.

Strain the liquid into a large mixing bowl and discard the solid ingredients. Put the liquid back into the saucepan, add the mushrooms, cover and simmer for 5 minutes.

Add the scallops and cook covered for 5 minutes longer. Strain into a large mixing bowl and set aside the scallops and mushrooms.

Put the liquid back into the saucepan and boil over high heat to reduce down to one cup. Be careful not to burn or discolor the liquid. Reserve this for the Parisienne sauce below.

Preheat the broiler.

Melt the butter in a saucepan over moderate heat. When the foam subsides, remove the pan from heat and add the flour. Return to low heat and cook, stirring constantly, for two minutes. Do not let the "roux" brown.

Remove the pan from the heat again and slowly pour in the reduced poaching liquid from the scallops and the milk, whisking constantly. Return the pan to high heat and cook, stirring constantly with a whisk. When the sauce thickens and comes to a boil, reduce the heat and simmer slowly for 10 minutes.

Mix the egg yolks and 1/2 cup cream together in a bowl, then stir into it 2 tablespoons of the hot sauce from above. Add 2 more tablespoons and whisk this mixture back into the simmering sauce.

Over moderate heat, bring the sauce to a boil, stirring constantly, and simmer for 30 seconds. Remove from heat and season with lemon juice, salt and pepper. The mixture should coat a spoon fairly thickly. If the sauce is too thick and pasty, add some more cream. Adjust the seasoning.

When almost ready to serve, use a sieve to drain the scallops and mushrooms of any liquid that may have accumulated under them and then return them to the bowl. Then pour about 2/3 of the cream sauce into the scallops and stir together gently.

Butter the ramekins (or scallop shell dishes), and spoon the scallop mixture into them. Cover with the remaining sauce and sprinkle with the grated Gruyere.

Place the ramekins on a baking tray lined with aluminium foil. Bake on top rack of oven just beneath the broiler for 5-6 minutes, or until the sauce begins to bubble and the top starts browning lightly. Remove as soon as this happens. (If the oven does not have a broiler, set the temperature to 550 degrees F)

Quickly garnish the ramekins with chopped Italian parsley, or a mixture of chopped Italian parsley and tarragon. To serve, set the ramekins on individual small plates and the plates themselves on larger dinner plates. If your small plates are not heat-proof, place a folded napkin under each ramekin.