Season the beef with salt and pepper. Melt a tablespoon of butter with a drizzle of the olive oil in a sauté pan until hot, then sear the beef on all sides. Remove from the pan to a board and let cool completely, then wrap and refrigerate.
In the same pan as the beef, prepare the mushroom duxelles: Melt the remaining butter and fry the shallots until translucent. Add the mushrooms, thyme, and bay leaf, and cook until very tender. Pour over the Madeira, bring to a boil, and cook until all the liquid has evaporated. Add the cream and cook down to a very thick paste. Season with salt and pepper. Stir through the parsley
Roll out one block of pastry to a rectangle large enough to fit the meat with a roomy border. Place on a baking sheet. Remove the filet from the fridge and unwrap. Spoon the mushroom mixture into the centre of the pastry and set the meat on top. Roll out the second sheet to fit over the whole fillet generously. Brush the margins of the bottom pastry with egg wash, then drape the second sheet over, pressing to seal well. Trim the edge to a 1-inch/2.5 cm border. Crimp the edges with your fingers. Refrigerate until ready to bake.
Heat the oven to 425°F/220°C. Brush the whole surface of the pastry with egg wash and make two slits in the top with a knife to allow steam to escape. Bake 15 minutes. Reduce the oven temperature to 400°F/200°C and continue to baking 20 minutes, depending on how well you like your meat done. Remove from the oven and let stand about 10 minutes before serving in slices.