1. Preheat oven to 425 degrees F. In a saucepan heat the butter, add the mushrooms and cook until wilted. With a wire whisk stir in the flour.
2. Meanwhile, bring the milk to a boil and add to the mixture, stirring vigorously until thickened. Add the seasonings, lemon juice and oysters with their liquor.
3. Line the bottom and sides of an 8-inch pan or small casserole with pastry, rolled to 1/8-inch thickness. Add the oyster mixture and cover with the remaining pastry. Seal the edges and cut gashes in the top to allow steam to escape. Bake until brown, about 30 minutes.