Heat the oil in a non-stick frying pan at medium high or in your pressure cooker on saute function and cook the onions until soft. Place the onions in the pressure cooker if using a fry pan or in a bowl if using the saute function on your pressure cooker.
Season the beef chunks with 1 tsp of salt and freshly cracked pepper. Add roughly 1 tbsp oil to the fry pan / pressure cooker and sear the beef chunks in batches. Once all the meat is seared, place into the pressure cooker (if using a pan). (Note 2)
While the frying pan / pressure cooker is still hot, pour in the Guinness. As it comes to the boil scrape the bottom of the pan to remove any browned bits. Pour the juices into the pressure pot (if using a pan).
Add the tomato paste, Worcestershire sauce, sugar, herbs and nutmeg to the pressure pot. Combine well and cook on high pressure / stew setting for 25 mins.
Carefully release the valve and depressurise as instructed in your user manual. Remove the lid and add the sliced mushrooms. Cook for 5 mins on saute setting.
While the mushrooms are cooking, preheat the oven to 180 C / 356 F and start to defrost one sheet of frozen puff pastry.
When the mushrooms are done, add 2 tbsp of cornstarch and 3 tbsp of cold water to a small bowl and combine well. Add it to the pot and stir until the gravy is nice and thick.
Check the seasoning of your pie filling and adjust it to your taste. Then turn your pressure cooker off.
Divide the pie filling between four large ramekins. Wet the rim of each ramekin, then cut 1cm wide pieces of puff pastry and press down over the rim (see picture for example).
Take the second sheet of frozen puff pastry so it can start to defrost.
Going back to the first sheet, cut out circles just slightly larger then your ramekins. Lay on top of each ramekin and use a fork to press down onto the pastry-lined rims. (Note 3)
Create an air vent by either cutting a cross in the middle, piercing the pastry a few times with a fork, or using a shape cutter to remove a portion of the pastry. Repeat with the remaining ramekins and second sheet of puff pastry. (Note 4)
Brush the pot pies with the beaten egg and then cook in the oven for 15 minutes at 180 C / 356 F fan forced. (Note 5)