Preheat your oven to 375 degrees F (190 degrees C). Heat the vegetable oil in a large, heavy-based casserole dish or Dutch oven over medium-high heat. Pat the beef cubes dry with paper towels and season them generously with salt and black pepper.

Sear the beef in batches until a deep, dark brown crust forms on all sides, taking about 5 to 7 minutes per batch. Transfer the browned beef to a plate and set it aside, keeping the remaining drippings in the pan.

Reduce the heat to medium and add the chopped onion and carrots to the pan. Cook for about 5 minutes, stirring occasionally, until the onions are soft and translucent. Sprinkle the flour over the vegetables and stir constantly for 1 minute to cook out the raw flour taste.

Slowly pour in the stout beer and beef stock, using a wooden spoon to scrape up all the delicious browned bits stuck to the bottom of the pan. Stir in the Worcestershire sauce and return the seared beef along with any accumulated juices to the mixture.

Bring the liquid to a gentle simmer, then cover with a tight-fitting lid and cook over low heat for 1.5 hours, or until the beef is meltingly tender and the gravy is thick and glossy. Remove the pan from the heat and let the filling cool slightly for 10 minutes before gently folding in the fresh oysters.

Transfer the beef and oyster filling into a 9-inch deep-dish pie pan or baking dish. Roll out the puff pastry sheet over the top of the dish, trimming any excess and crimping the edges tightly against the rim to seal. Brush the pastry evenly with the beaten egg and cut a small slit in the center to allow steam to escape.

Bake for 30 to 35 minutes until the pastry is puffed, crisp, and deep golden brown. Let the pie rest for 10 minutes before serving hot with a side of creamy mashed potatoes and buttered peas.