Cut the meat into small cubes.
Brown the onions in butter, cover, and let them steam until tender but crisp.
Fry out the bacon pieces and transfer to absorbent paper.
Roll out the pastry and put a strip 2 inches wide around a 2-quart baking dish, slightly overlapping the top edge.
In the dish place alternate layers of beef, onions, bacon, oysters if wished, and sauce.
Dampen the pastry strip, then cover the dish with a top crust, pressing it against the strip to seal it and turning it under. Crimp the edges and make a vent in the center of the crust.
Brush the pastry well with the egg and cream beaten lightly together.
Bake at 425°F for 10 minutes. Reduce the heat to 375°F and cook until the crust is done and the meat is thoroughly cooked - another 35 to 40 minutes.