Heat oven to 325˚ F.
Cut the beef into 1 1/2 in cubes and season. Heat half the butter with the oil in a frying pan and sear the meat over high heat. (When you add a little oil to butter, it raises the smoke point so you can fry things at higher temperatures.) Do this in batches, because if the pan is overcrowded, the meat will stew rather than sear. Transfer the meat to a plate, add the onions to the pan and cook for 5–6 minutes, stirring occasionally. Add the flour, stir and cook for 1 minute. Blend in the stock, add the meat and bring to a boil. Transfer to a casserole, cover and bake for 1 1⁄2–2 hours.
Meanwhile, sauté the mushrooms over high heat in the rest of the butter, season, and keep aside. Open the oysters (with an oyster knife) and put in a bowl with their juices.
When the beef is tender, thicken the meat juices in the casserole slightly with roux (if necessary). Add the mushrooms, oysters, and the oyster juice to the casserole. Bring back to a boil for 2–3 minutes and taste for seasoning. Let cool.
Increase the oven temperature to 475º F. Put the entire mixture into a deep pie dish and cover the top with pastry (see image). Then brush with egg wash and cook in the oven for 10 minutes (this quick burst of heat rises the pastry). Then reduce the oven to 350ºF and cook for another 15–20 minutes, or until the pastry is puffed and golden.
Excerpted with permission from Forgotten Skills of Cooking: 700 Recipes Showing You Why the Time-Honored Ways Are the Best by Darina Allen. © Darina Allen. Published by Kyle Books. Photographs © Peter Cassidy.