For the pastry, tip the flour and mustard powder into a bowl, then rub in the butter and lard (if using) with a large pinch of salt until the mixture resembles breadcrumbs. Pour in half the egg and 75ml ice-cold water. Use your hands to work the mixture into a rough dough, then knead for a minute or so to bring it together. Cover and chill until needed. Will keep chilled for up to a day.
For the filling, bring a pan of salted water to the boil, then add the eggs and cook for 7 mins 30 seconds. Lift out using a slotted spoon into a bowl of iced water. Once cool, peel the eggs, then trim away some white from the sides (so that every slice of pie gets a bit of yolk), then set aside. Tip the sausagemeat into a bowl with the bacon, thyme, pickled onions and garlic, then squash everything together with your hands and season. Will keep chilled for up to a day.
To assemble, heat the oven to 200C/180C fan/gas 6 with a sturdy baking tray inside. Roll two-thirds of the pastry out to the thickness of a £1 coin on a lightly floured surface and use it to line the base of a 20cm round loose-bottom or springform tin, tucking the pastry into the corners and leaving an overhang. Sprinkle half the stuffing mix over the base, then pack in a third of the sausage mixture. Arrange the eggs in a circle on top with the cut sides touching. Pack over the rest of the sausage mix, filling in any gaps, and scatter over the remaining stuffing.
Brush the edges of the pastry with some of the remaining egg. Roll out the last third of pastry to fit the top of the pie. Drape it over, press the edges together, trim away the excess and crimp with your fingertips. Poke a hole in the middle of the pie, score with the back of a knife and use the excess pastry to make pastry leaves for decoration. Brush the top all over with a little more of the egg, then bake for 50 mins or until a digital cooking thermometer reads over 70C when inserted into the middle. Brush all over with the last of the beaten egg and bake for another 10-15 mins until deep golden. Leave to cool completely in the tin. Serve with salad and piccalilli, if you like.
