Susannah Shier's steak and kidney pie

Laura Chapman is no great cook, but thankfully her sister Susannah is, and has agreed to share some recipes with us. The first is a classic steak and kidney pie, inspired by Ruth Watson at BBC Good Food. Serve with buttery mash and greens for proper winter comfort food factor.
 Ingredients:
1 Native Beef ox kidney, about 400g/14oz
1kg trimmed Native Beef diced beef
250g mushrooms
1 large onion
About 3 tbsp oil (I used rapeseed, any vegetable oil will do)
A knob of butter
50-85g plain flour, depending on how thick you like your gravy
About 600ml beef or vegetable stock
1 bay leaf
Salt and black pepper
1 packet puff pastry
A little milk or beaten egg (for glazing)
Method:
1.Preheat the oven to 160C/gas 3/fan 140C. 
2.Use a sharp knife to cut button-mushroom-sized lobes of kidney from the white central core (discard the core). Cut the beef and mushrooms into bite-sized pieces, peel and roughly chop the onion. 
3.Heat 1 tablespoon of the oil in a large, heavy frying pan over a medium-high heat. Throw in the kidney and fry until lightly coloured. Watch out as it may spit a bit. Tip into a colander to drain off any excess liquid.
4.Wipe out the frying pan and return it to a low-medium heat, adding the knob of butter and a little more oil. Lightly sauté the onion and mushrooms. Transfer to a large casserole, using a slotted spoon.
5.Toss the beef in the flour and season it with the salt and pepper. Return the frying pan to a medium high heat, adding a little more oil and butter if needed. Shake off any excess flour (reserving it) then fry the beef in batches until golden-brown. As each batch is done, transfer it to the casserole. 
6.Add the drained kidneys to the beef, mushrooms and onion in the casserole. Pour over the stock (so the contents are just covered), throw in the bay leaf and the excess flour from before. Stir, cover with the casserole lid and cook in the oven for 75-90 minutes until the meat is tender and the sauce is thick. Cool thoroughly.
7.Roll the pastry out thinly on a well floured surface. Invert a pie dish on to the pastry, then use the dish as a guide to cut out the pastry lid, adding an additional 1cm all round. From the remnants, cut out some extra strips of pastry – these are to go around the rim of the dish (and ideally about halfway down the inside), to form a seal for the pie lid.
8.Lightly butter the rim of the pie dish and line it with the strip(s) of pastry, sealing any joins with a little dab of water. Spoon in the meat mixture to come level with the top of the dish. Don’t overfill: reserve any excess gravy to serve hot with the pie.
9.Brush the pastry rim with a little water, then drape the pastry lid over it, pinching the edges to seal. Cover with cling film and keep in the fridge if not baking immediately.
10.An hour before serving, preheat the oven to 200C/gas 6/fan 180C. Make four slashes in the lid of the pie, then brush with milk or beaten egg. Bake for 35-45 minutes until the pastry is golden, turning the heat down 10º after about 20 minutes.
Serve and enjoy!