Preheat the oven to 200°C.
In a large, heavy based saucepan, heat the oil. Put the flour into a large oven tray and lightly dust the beef pieces, shaking off and reserving any excess flour. When the oil is smoking hot, cook the meat in batches so as not to crowd the pan. When evenly seared, remove each piece from the pan and place on a plate. Repeat the process until you've seared all the meat. Add butter to the pan, then add onions, thyme and garlic. Sweat over a low heat until translucent but not coloured. Turn up the heat. Add the mushrooms and the reserved flour, stirring thoroughly. Once the mushrooms are softened, transfer the meat back into the saucepan along with the beef stock, salt and pepper. Bring to boil. Turn down the heat, cover and simmer for about 1.5 hours or until the meat is tender. Once cooked, use a slotted serving spoon to transfer all the meat and vegetables from the saucepan to a pie dish. Bring the liquid in the saucepan back to boil and reduce to about 400ml remaining. Add this to the meat and stir through thoroughly. Set aside and allow to cool.
Once completely cooled, add the oysters to the pie dish and cover with the sauce. Roll out the puff pastry on the bench and cut around the edges, making sure that the pastry is about 1cm wider than the dish. Lay the pastry over the pie dish and crimp the edges. Cut a cross in the middle of the pie to allow steam to escape when cooking. Brush the pastry with the beaten egg and cook in the oven for about 30–45 minutes. Serve with creamy herb mash potato.
Goes perfectly with GUINNESS® Foreign Extra Stout