Soak the kidney overnight in water with the vinegar and salt in the fridge. Replace the soaking solution as often as you like.
Remove and discard the white membranous material from the kidney, and dice the glumpy lumps into small 1/2-inch cubes and get them into a bowl.
In a big pot over medium-high heat, heat some oil and start to brown the cubed beef in small batches, removing the beef when it is browned onto a large plate or casserole dish. The meat doesn’t need to be cooked through, just browned on the outside for flavour. After you have browned all the beef, brown little batches of the kidney (or mushrooms), and then transfer to the same dish as the beef.
Reduce the heat to medium, pour in some oil if the drippings don’t amount to about 2 Tbsp, and cook the onion until it is translucent, about 5 minutes. Sprinkle in the flour evenly and mix it up a bit with a wooden spoon. Crank the heat to full blast and throw in the beef broth and the Guinness. Scrape the sticky bits off the bottom of the pan. Now add all the previously browned beef along with their juices, and the kidneys (or mushrooms), Worcestershire sauce, nutmeg, and salt and pepper. Drop the heat to low, cover and simmer for at least 2 hours.
Spoon the mixture into a baking dish that will accommodate it, and top with the pastry - no need to cut it to shape, just drape it over the top and let the edges hang over. Press them against the sides of the dish. Bake at 400F for about 15 minutes, until the pastry is golden and it's all heated through.