To make the pastry, mix the flour, mustard powder, lard and butter with a generous pinch of sea salt until completely combined, then gradually add up to 250ml ice-cold water to make a soft dough. This can be done in a food processor if you prefer. Knead the thyme leaves into the pastry, then cover and leave to rest in the fridge for at least 1 hr. Can be made up to two days ahead and kept chilled, or frozen for up to a month.
Heat the oven to 220C/200C fan/gas 7 and put a baking tray in the oven. Heavily butter a 26-28cm pie dish and dust well with flour. Cut off a third of the pastry and set aside. Roll out the remaining pastry to a thick round that will easily line the pie dish with an overhang, then line the tin. Tip in the beef mixture. You want the filling to be slightly higher than the rim of the dish. Brush the edges with egg yolk. Roll out the remaining pastry to a thick round, big enough to cover the pie, then lift onto the pie dish. Trim the edges, crimp together, then re-roll the trimmings and cut into decorative shapes, such as leaves. Brush the top with more yolk. Make a hole in the centre of the pie with a knife, and put it on the hot baking tray.
Bake for 20 mins, remove, brush again with yolk, scatter with sea salt and bake for 20-25 mins until golden. Leave to rest for 10 mins while you heat up the extra gravy.