My local butcher started selling pig kidneys so I bought one to make this traditional British pie. It was delicious! The filling should simmer a couple of hours but it's very easy to make with premade puff pastry.
Dice the meat from the kidney, discarding the fibrous core. Soak the meat in milk in the fridge at least an hour.
Heat 2 tablespoons oil in a heavy saucepan and saute the onion over low heat until softened. You can keep the onion simmering while you prepare the other ingredients, adding them to the saucepan as you go.
Heat 1 tablespoon butter and 1 tablespoon oil in a fry pan over high heat and brown the kidney pieces (discarding the milk). Add them to the saucepan with the onions and wipe out the frypan.
Add a little more oil and 1 tablespoon butter to the frypan and cook the mushrooms, then add them to the saucepan.
Toss the stew meat with the flour, add a little more oil and butter to the frypan and brown the stew meat, then add to the saucepan with the onions as well.
To the saucepan, add the beef stock, the bay leaf, the Worcestershire sauce, and a few grinds of pepper. Hold off on the salt - stock is often pretty salty already. Add a little more flour if it looks very thin. Simmer, covered, over low heat for 2 hours. Now you can check for salt!
When the pie filling is nice and thick (you might want to remove the lid and reduce it a bit) take it off the heat while you prepare the pie.
Preheat the oven to 190 degrees C. Let the puff pastry thaw until it is just manageable. Oil a 23cm pie pan and lay in the first sheet of puff pastry. Add in the pie filling (remove the bay leaf). Top with the second sheet of puff pastry and seal and trim the edges.
Brush the pie with beaten egg and poke a few holes in the top to let steam escape. Bake 30 minutes or until brown and puffed. Serve with green peas!