Prepare a puff paste and lay on the ice to cool. Boil the Oysters in their own liquor. Drain, put the butter in the saucepan, and when it is heated, add the grated onion and rub in the flour until smooth. Add a gill of mushroom juice and pepper, salt and Cayenne to taste and the mushrooms chopped in quarters. Then add the Oysters and let all stew about five minutes, adding the lemon juice.