Preheat the oven to 160°C/fan 140°C/gas mark 3. Pat the meat dry with kitchen paper. Put the flour on a large plate or tray, then add the mustard and season. Coat one-third of the meat in the flour. Heat the oil in a large, non-stick frying pan until hot and fry the floured meat over a medium-high heat. Remove the meat from the pan using a slotted spoon and transfer to a flameproof casserole dish. Divide the remaining meat into 2 batches and repeat the flouring and browning, adding more oil if needed.
Add the onion to the frying pan, with a little more oil if necessary, and fry for 3 minutes or until golden brown, stirring frequently. Add the celery and carrots and stir to mix, then fry for 2 minutes before tipping the vegetables on top of the meat in the casserole.
Pour the Guinness and stock into the casserole dish and add the redcurrant jelly. Mix well and bring to the boil, stirring. Add the bay leaves and season. Cover the casserole and cook for 2½ hours or until the meat is tender. Remove from the oven, taste the gravy for seasoning and leave until cold.
Preheat the oven to 220°C/fan 200°C/gas mark 7. Roll out the pastry and cut out a lid and a strip for the lip of the pie dish. Remove the bay leaves from the casserole and stir in the parsley and thyme, then transfer the meat and vegetables to the pie dish using a large spoon, together with enough of the gravy to come just below the lip of the dish. Brush water around the lip of the dish then cut the pastry strip into smaller pieces and place the strips on the moistened lip. Moisten the strips, cover with the lid and press to seal.
Trim and scallop the edge, then brush the pastry lid with beaten egg to glaze. Use the trimmings to make decorations and re-glaze with as much of the remaining egg as needed. Cut a small slit in the centre of the lid.
Bake the pie for 30-35 minutes or until the pastry is risen and golden brown. If you have any gravy left over, reheat until bubbling, pour it into a jug, and serve alongside the pie.
Mary Berry Cooks The Perfect by Mary Berry (Dorling Kindersley, £25)