1. Make the filling a day ahead if you can as it takes a couple of hours to cook and it needs to be completely cold before filling the pie – at least a couple of hours in the fridge.
2. To make the pastry – that can be made well ahead as well – stir the flour, salt and mustard powder in a large bowl or in the bowl of a standing mixer. Add the diced butter and lard and process or rub with your fingers until the mixture resembles coarse breadcrumbs. Beat the egg with the water and vinegar and gradually add to the flour mixture, stirring the liquid in with a large fork or a spatula. Add a little more water if it looks dry. When it starts clumping together, give it a short knead with your hands and gather the pastry into a ball. Divide it into 1/3 and 2/3; wrap both portions in cling film and chill for at least an hour.
3. To make the filling, dice the steak into 2cm cubes. Sprinkle with salt and pepper, rub the seasoning in and dust with 1 tbsp. of flour. Stir the meat to coat it evenly.
4. Heat the oil in a large flame- and oven-proof casserole (or use a skillet and then transfer everything to an oven dish) with a lid. Brown the meat all over, remove and put aside. Add the butter into the casserole, add the onions and garlic and cook for 5 minutes until softened and lightly browned.
5. Crumble the dried mushrooms into the onions, add the chopped fresh mushrooms and cook for another 5 minutes. Preheat the oven to 150C/300F/gas 2.
6. Mix the bay, thyme, sugar, cocoa and vinegar in a jug with half the amount of beer. Return the meat to the casserole, sprinkle with the other tbsp. of flour; pour in the flavoured beer and add more if necessary, so the meat is just covered. Bring it to a simmer, cover with a lid and transfer into the oven. Cook for 2 hours, stirring two or three times and topping up with more beer if the liquid cooks off too much.
7. If there’s too much gravy, scoop the meat out with a slotted spoon or decant some gravy into a bowl; transfer the filling to a clean bowl so it cools down quicker, if you’re making the pie on the same day. Cover and chill in the fridge.
8. Prepare the pastry bottom and lid: roll out the bigger portion to a round of about 24cm in diameter, depending on the size and shape of your pie dish; it needs to cover the bottom with an overhang. Break the egg that will be used for brushing and use some of the white to brush over the bottom of the pie, to stop it from getting soggy. Return to the fridge while you roll out the lid to a smaller circle; just to cover the top.
9. Preheat the oven to 190C/200F/gas 5. Spoon the filling into the pastry lined pie dish; brush the edges with the egg lightly beaten with a spoonful of cold water.
10. Cover with the pastry lid, press the edges and trim the excess pastry; you may want to cut out decorations to place on top of the pie. Scrunch or crimp up the edge of the pie and cut a few slits for the steam to escape. Brush it generously with the beaten egg.
11. Bake the pie in the lower half of the oven for about 45 – 50 minutes until golden brown.
12. Serve immediately with the decanted extra gravy on the side, mashed potatoes and green vegetables or a plain green salad.