Season the beef well with salt and pepper, then brown in a heavy casserole until deeply coloured and lift out.
In the same pan fry the pancetta until crisp, add the mushrooms and cook until golden.
Stir in the onions, garlic, rosemary and tomato paste, cooking until softened and fragrant.
Pour in the ale and beef stock, bring to a simmer and return the beef to the pan.
Cover and cook at 130°C for about 5 hours until the beef is tender.
Remove the beef and break it up into large shreds.
Stir the cornflour slurry into the sauce to thicken slightly, then add the mustard and fold the beef back in.
Let it cool completely before stirring through the chopped parsley.
For the pastry, rub the butter into the flour with a pinch of salt until it resembles breadcrumbs.
Stir in the egg yolk and just enough cold water to bring it together.
Wrap and chill for 30 minutes.
Heat the oven to 200°C.
Roll out two-thirds of the pastry and line a pie dish.
Fill with the cooled beef mixture, roll out the remaining pastry for the lid, and seal the edges well.
Brush with beaten egg and cut a small vent in the centre.
Bake for 30–40 minutes until golden and crisp.
