In a large pot, heat a tablespoon of oil over medium-high heat. Sear the beef chuck roast for about 4-5 minutes on each side until it's nicely browned. Add chopped onions, carrots, parsnips, celery, and fresh thyme. Pour in beef stock to cover the ingredients, bring to a simmer, and then cover. Allow it to simmer gently for about 2.5 hours, stirring occasionally, until the beef becomes tender and flavorful.

Once the beef stew filling reaches tenderness, remove the pot from heat and let it cool for 10-15 minutes. Transfer the filling into a baking dish, spreading it out evenly.

While the filling cools, preheat your oven to 375°F (190°C). Roll out the thawed puff pastry on a lightly floured surface until it’s about 1/8 inch thick. Drape the pastry over the beef filling in the baking dish, trimming any excess edges. Make a few slits on top to allow steam to escape.

In a small bowl, whisk an egg and brush it over the puff pastry. Place the baking dish in the preheated oven and bake for approximately 30 minutes until golden brown. Remove from the oven and let it cool for a few minutes before serving.