Pat the beef chuck as dry as possible and place in a large bowl. Season generously with salt and pepper.
Place a pot over high heat and add half of the olive oil. Sear half of the beef for 5–6 minutes, until deeply browned. Remove and repeat with the remaining batches, then remove all the beef from the pot.
In the same pot over medium-high heat, add the onion, celery, carrot, and a pinch of salt. Sauté for 6 minutes. Add the garlic and sauté for 1 minute. Add the flour and stir to coat the vegetables, cooking for 1 minute to cook out the raw flour taste. Gradually pour in the beef stock, mixing well to prevent lumps.
Add the tomato paste, beef stock cubes, red wine, Worcestershire sauce, and bouquet garni. Season well with salt and pepper, mix, and bring to a boil.
Add the beef and any resting juices, mix well, and bring back to a boil. Cover, reduce the heat to low, and cook for 2 hours. Remove the lid and discard the bouquet garni. Check the seasoning and simmer for a further 20 minutes, or until the beef is falling apart. Remove from the heat and let cool — this is best done overnight, or in about 4 hours in the fridge until lukewarm.
Preheat the oven to 200°C (400°F). Place the cooled beef mix into the blind-baked pastry shells to assemble the pies. Cut the puff pastry slightly larger than the pie moulds. Brush the edges of each pie mould with the beaten egg and place on the puff pastry lids. Brush the tops with the remaining egg and score if desired. Sprinkle with sea salt flakes and cracked black pepper.
Bake for 18–20 minutes (small pies), 25–28 minutes (medium pies), or 30–35 minutes (large pies), until golden brown — if making large pies and the pastry browns faster than the centre heats through, loosely tent with foil for the last 10 minutes. Remove and let cool for 15–20 minutes before serving.