Lobster a la Riseholme

Fill a heavy cook-pot with enough water so the lobsters will be completely covered when plunged into it. Lightly salt the water, bring to a boil and immerse the lobsters. Allow the water to return to a boil. Reduce to a simmer; cook about 11 minutes.
Drain the pot and plunge the lobsters into cold water. Remove meat from the tail, body, and claws. Discard the sandbag, the intestinal vein and lungs. Reserve the green tomalley (liver) and roe.
Dice meat into bite-sized pieces and combine with cooked shrimp.
To prepare a white sauce or béchamel, melt the butter over low heat, blend in the flour and cook over low heat for 4 minutes, slowly stirring in the milk. Add the onion, studded with the cloves, plus the bay leaf. Cook, stirring until thickened and smooth.
Place in oven at 350 F for 20 minutes, then strain.
Beat the egg yolk and cream until blended. Add a little of the sauce to the egg yolk and cream, stirring constantly, then return the mixture to the sauce. Add the grated Parmesan and Gruyère. Keep stirring with a small whisk until cheese has melted and blended. Season lightly with salt and a few grains of cayenne. Add 1 teaspoon of prepared mustard, the sieved tomalley and roe, and 2 tablespoons of sherry.
To serve, fill the tails with one-third of sauce, add seafood and cover with the rest of the sauce. As a final touch, sprinkle the top with a mixture of grated Parmesan and melted butter. Put under the broiler until the sauce is golden brown. Serves six

Lobster à la Riseholme