Put the olive oil, butter and herbs into a large casserole-type pan (roughly 24cm in diameter and 12cm deep) on a high heat. Roughly chop and add the onions, with the diced meat, the shin bone and a couple of pinches of salt and pepper. Mix well, and cook for 10 minutes, stirring occasionally. Add the tomato purée, stout, flour and stock and stir until everything comes together to a simmer. Turn the heat down very low, pop the lid on and let it cook for 1 hour, stirring occasionally. When the hour is up, stir in the pearl barley. Put the lid back on and simmer for another hour, then remove the lid and simmer for a further 30 minutes, or until the meat shreds easily and the gravy is thick. Spoon away any oil from the top, then stir in the mustard and Worcestershire sauce and finely grate in the cheese. Season to taste. While the stew is ticking away, put the flour, suet and butter into a bowl with a good pinch of salt. Use your thumbs and forefingers to rub the butter into the flour until it resembles cornflake shapes. Lightly stir in 125ml of cold water, then use your hands to gently pat and push it together into a rough dough. Do not overwork it. Wrap the dough in clingfilm and put into the fridge until needed.
Preheat the oven to 180°C/350°F/gas 4. Discard the shin bone and ladle the hot meaty stew into the pie dish (24 x 30cm, and about 4cm deep, is about right). Use some of the beaten egg to eggwash the edges of the pie dish, then dust a clean surface and a rolling pin with flour and roll out the pastry about 1cm thick and a little bit bigger than your pie dish. Carefully place on top of the pie, then trim off any overhanging pastry. Pinch and squash the edges of the pastry to the dish. Eggwash the top, and cook the pie right at the bottom of the hot oven for around 45 to 50 minutes, or until your pastry is golden and gorgeous. Serve with steamed drained greens.