1) Rub the fillet with salt and pepper and brush lightly with oil. Heat 1 tablespoon oil and the butter in a large skillet over medium-high heat. Place the fillet in the skillet and lightly sear all surfaces of the meat, including the ends. Remove the meat from the pan and set aside.
2) Add more oil to the pan, if necessary, and then add the shallots and garlic and saute for 2-3 minutes. Add the mushrooms and saute for 2-3 minutes longer. Add the oysters and sherry and cook until the oysters are opaque. Remove from the heat and allow to cool slightly. Drain off any liquid and combine the mixture with the breadcrumbs in the bowl of a food processor. Process until well blended and nearly smooth. Line a colander with several layers of cheesecloth and spoon the pate into it. Allow to drain for 10-15 minutes. If the pate seems too wet, pick up the edge of the cheesecloth, twist it together and wring out the excess liquid.
3) Roll out the puff pastry dough on a lightly floured surface to form a rectangle less than 1cm thick. Spoon some pate into the centre of the dough and spread it to form a shape similar to that of the fillet. Place the meat on the pate then cover the top and sides of the meat with the remaining pate. Wrap the pastry around the fillet, overlapping the edges to form a seam. Moisten the edges with water and pinch lightly to seal. Trim the top layer of pastry from both ends and fold the ends up over the sides of the meat, sealing as before.
4) Carefully turn over the pastry-wrapped fillet and place seam side down on a lightly greased baking sheet. Brush pastry with the beaten egg. Decorate the top with designs cut from the pastry scraps, if desired. Brush the cutouts with beaten egg.
5) Bake the fillet for 15-20 minutes, or until the pastry is golden brown. Remove from the oven and let stand for 10 minutes before slicing.