Preheat oven to 150c (130c if fan forced). In a large oven-proof pot, heat olive oil on medium-high heat. Sear brisket on all sides until browned with crispy edges. Set aside. In the same pot, add in onion, carrots, and celery. Season with salt and pepper and fry, stirring occasionally, until caramelised on the edges.
Add in thyme, garlic and cook for an additional minute before adding in stout to deglaze the pan and reduce. Pour in the stock and a dash of white wine vinegar. Bring to the boil, then cover and remove from heat and place into preheated oven. Roast for 2.5-3 hrs or until brisket is tender enough to be pulled apart with a fork, but still holding its shape. Remove the meat from the liquid and shred with two forks into rough chunks.
Pour the stock through a sieve, discard solids. Set aside. Wipe down your pot to clean, and heat olive oil and butter on low heat. Add in onions and a teaspoon of salt and stir occasionally to prevent from burning. Allow the onions to sweat and become translucent. After about 10-15 minutes, they will start to stick to the bottom of the pot. Add in garlic and turn up the heat to medium, adding in a ladleful of the strained stock to deglaze and cook until stock has reduced and onions are soft, taking on a rich caramel brown colour and have shrunken significantly. Remove from heat and set aside.
Turn the heat up to med-high and in shiitake mushrooms. Fry for 2 minutes or until cooked through. Add the caramelised onions back in, followed by the flour, one tablespoon at a time. Stir, then add in approx. 2.5-3 cups of the reserved liquid, stirring until the liquid has thickened into a loose gravy. Add in the shredded brisket and cook until the gravy has thickened and coats the back of a spoon. You may need to adjust with more flour or stock.
Preheat oven according to puff pastry package directions. Transfer the pie filling to individual ramekins. Cut pastry to size, and drape over the ramekins. Seal the edges with the back of a fork, brush with egg wash and cut a slit at the top to let the steam out. Bake according to package directions. It should be golden brown. Serve with freshly chopped parsley and enjoy.
I recommend allowing the filling to cool prior to adding the pastry on top. The pastry must be chilled to allow it to puff up, and if the filling is too hot it can weld the layers together, resulting in a soggy pie! Because of this, you can make the filling ahead of time and add it to ramekins when needed. It’s very good to freeze, or keep in the fridge for up to two days.