Heat the butter in a small pan and cook the onion over a low heat for 5 mins until soft but not coloured. Tip all the filling ingredients, except the pickled onions, into a large bowl and mix well into a large meatball – the easiest way of doing this is getting your hands in and scrunching the mixture through your fingers. Divide the mix into six equal balls and press a pickled onion into each, making sure the onion is completely encased. Chill in the fridge for 1 hr or so.
Meanwhile, make the pastry by putting all the dry ingredients in a stand mixer with a paddle attachment. Bring 100ml water, the lard and ½ tsp salt to the boil, pour onto the dry mix and bring the pastry together.
Heat oven to 200C/180C fan/gas 6. Roll the pastry out to the thickness of a £1 coin. Cut out six 12cm circles and use each to line a hole in a greased muffin tin. Re-roll the trimmings, cut six 8cm circles for the lids and set aside. Press a meatball into each pastry-lined muffin hole. Brush the edge of each with a little beaten yolk, place a lid on top, crimp the sides together and poke a hole in the middle of each. Glaze the pie tops with egg yolk and bake for 20 mins until golden brown. Leave the pies to cool completely before eating. Can be made two days ahead and kept in the fridge.