Melt butter and saute onions until golden brown. Stir in flour and cook, stirring, until the mixture is lightly browned. Gradually stir in stock, wine , mushrooms, and shrimps. Cook slowly over low heat, stirring constantly, for 10 minutes. Open oysters and put them in their deep shells on individual ovenproof baking dishes. A bed of rock salt will help the shells stand firmly upright. Bake the oysters in their own juices in preheated moderate oven (350F) for about 6 minutes. Beat egg yolks with cream and beat hot sauce into the mixture. Reheat the sauce but do not boil. Cover each oyster with some of the sauce and sprinkle lightly with equal parts of dry bread crumbs and parmesan cheese. Return the oysters to the oven for about 10 minutes, until the topping is browned.