First, make the pastry. Put the flour, mustard powder, ½ tsp salt, the lard and butter in a food processor and pulse until combined. Add 1 egg and pulse again. Add 1 tbsp cold water, if needed, until it forms a short pastry. Knead quickly into a ball, then wrap and chill in the fridge for at least 30 mins. Can be made three days ahead or frozen for two months.
Season the steak generously and toss with the flour and mustard powder. Heat half the oil in a large pan. Brown the meat in a few batches, adding more oil as you need to and making sure you don’t overcrowd the pan. Turn the meat only when the underside has a nice dark crust. Transfer the chunks to a plate as you go.
Add the bacon to the pan and fry until beginning to crisp. Tip in the carrots, onions and bay leaves and cook for another few mins until the veg is soft. Stir in the Worcestershire sauce and tomato purée for 1 min, scraping the tasty bits from the bottom of the pan, then add the stock, thyme and browned beef. Season, cover the pan and simmer gently for 2½ hrs, stirring now and again to prevent the sauce from sticking, until the beef is tender. Remove the lid for the last 30 mins to thicken the sauce. Alternatively, braise in the oven at 160C/140C fan/gas 3 for the same amount of time. Leave to cool. Can be chilled for up to three days, or frozen for two months.
Transfer the beef to a pie dish and crumble the cheese over the top. Place the marrowbone in the centre, nestling it into the meat. Alternatively, use a pie funnel or piping nozzle. Roll out the pastry on a floured worktop until large enough to cover the pie and about ½cm thick. Brush the remaining beaten egg over the edges of the pie dish, drape the pastry over, trim the excess and pinch around the edge to seal. Cut a hole for the marrowbone or pie funnel to poke through, and brush the top all over with beaten egg. Use the pastry trimmings to decorate the pie however you like. Chill for about 30 mins while you heat the oven to 200C/180C fan/gas 6.
If you have any beaten egg left, give the pie a final brush for extra glossiness. Bake for 45-50 mins until the pastry is deep golden.