Heat the olive oil in a large oven-proof pot or Dutch oven over medium-high heat. Sear the beef cubes in batches until browned on all sides, about 5-7 minutes per batch, then transfer to a plate.

In the same pot, add the chopped onion and cook until softened and translucent, about 5 minutes. Stir in the minced garlic and cook for an additional 30 seconds until fragrant.

Return the beef to the pot and sprinkle the flour over the meat and onions. Stir well and cook for 2 minutes to gently toast the flour and coat the ingredients.

Pour in the stout beer and beef broth, scraping up any browned bits from the bottom of the pot. Stir in the fresh thyme and black pepper, then bring the liquid to a steady simmer.

Cover the pot, reduce the heat to low, and let it simmer gently for 1 hour, or until the beef is very tender and the gravy has thickened. Remove from heat and let it cool slightly.

Preheat your oven to 400°F (200°C). Transfer the slightly cooled beef filling to a 9-inch deep pie dish and gently stir in the drained oysters.

Roll out the thawed puff pastry sheet to fit over your pie dish. Place the pastry over the filling, crimp the edges to seal against the dish, and cut a few small slits in the center of the dough to let steam escape.

Brush the top of the pastry evenly with the beaten egg wash. Bake in the preheated oven for 25-30 minutes, or until the pastry is puffed and deep golden brown. Let the pie rest for 10 minutes before serving alongside mashed potatoes or buttered peas.