Heat a glug of oil in a large casserole dish or deep ovenproof saucepan. Add the onions and sweat until soft. Now add the garlic, celery and mushrooms. Cook out for 5 minutes.
Add the stewing steak and allow to brown on all sides (don't turn it too much, you want a nice colour to form).
Once the steak has browned, add the salt, pepper, mustard powder, rosemary and flour. Stir to coat then pour in the Guinness and top up with water until the meat is covered. Pop the lid on and place in the oven. Cook for 1 1/2- 2 hours or until the meat is tender and the sauce thickened.
Increase the oven temperature to 200c.
Once the filling is ready, spoon into individual pie dishes. Top with a circle of puff pastry and crimp the edges. Brush with egg and bake for 20-25 minutes. Enjoy with mash and green vegetables.