1 large free-range egg yolk, mixed with a little milk
Heat the oven to 180°C/160°C fan/gas 4. Heat 2 tbsp of the oil in a large frying pan and brown the meat in 2-3 batches (adding more oil if necessary). Set each batch aside.
Meanwhile, heat 2 tbsp more oil and the butter in a large casserole, add the onions and some salt and pepper and fry over a medium heat for 5 minutes. Add the carrots and celery and cook for 10 minutes. Stir in the flour and mustard powder, then cook, stirring, for 2-3 minutes.
Add the browned meat to the veg, then add the stout, vinegar, jelly, a good grating of nutmeg, the thyme and some salt and pepper. Stir, cover with the lid, then cook in the oven for 1½ hours. Uncover, then cook for 1 hour more until tender. Take out and leave to cool (see Make Ahead).
Turn up the oven to 200°C/180°C fan/gas 6. Divide the cooled pie illing among the dishes (or put in one large dish; see tip). Roll out the pastry on a floured surface to the thickness of a £1 coin, then cut out circles 2.5cm larger than the dishes.
Lay the pastry over the filling then, leaving a slight overhang, crimp the edges and use the tines of a fork to seal the edges well. Brush the pie tops with the beaten egg and milk, then make a hole in the middle of the pastry to let the steam escape.
Bake for 30-35 minutes until the pastry is a rich hazelnut brown. Serve with mash and greens.