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.
To make the open Pates so much used at luncheons, and entertainments in New Orleans, cut the puff paste into round cakes. Those intended for the bottom crust should be about a little less than an eighth of an inch thick. Those intended for the upper layers should be a little thicker. Take a small biscuit cutter and remove a round paste from the center of these latter. This will leave a nice ring. Carefully place this upon the bottom crust, and then in a second ring, until the cavity is deep enough to hold several Oysters. Lay the pieces that have been extracted into a pan with these and bake to a fine brown in a quick oven. Take out and fill the cavities with the Oysters prepared as above, fit on the top lightly, set in the oven a second or two and serve.