Begin by slicing your beef chuck roast into medium-large pieces, then season with salt and pepper. Heat a sturdy pot over medium heat and add a splash of oil. Once shimmering, add the beef pieces and brown them for about 5–7 minutes until they're nicely seared on all sides.

Next, stir in chopped onions, celery, diced carrots, and parsnips into the pot with the beef. Cook the mixture for about 5–8 minutes, stirring occasionally until the onions become translucent and the vegetables are slightly tender.

Pour in enough beef stock to cover the beef and vegetables, and sprinkle in fresh thyme or dried thyme if using. Bring the mixture to a gentle simmer, then cover the pot and let it cook for approximately 2.5 hours on low heat.

Once the stew has cooked, remove it from the heat and allow it to cool slightly. If you're preparing ahead of time, transfer the filling to a bowl and refrigerate it for a couple of hours or overnight.

While the filling cools, thaw your puff pastry according to package instructions. Once ready, roll it out slightly on a floured surface to fit your baking dish. Place it over the cooled stew filling, tucking it in at the edges and cutting slits into the pastry top.

Preheat your oven to 400°F (200°C). Brush the top of the pastry with an egg wash. Place the Beef Pot Pie in the oven and bake for about 30 minutes, or until the pastry is puffed and golden brown.