1. Preheat your oven to 400°F (200°C) and prepare a pie dish.
2. In a large skillet, heat the olive oil over medium-high heat. Season the beef cubes with salt and pepper, then sear them in the hot oil until browned on all sides. Remove the beef and set aside.
3. In the same skillet, add the diced onion and garlic. Sauté until the onions become translucent, about 5 minutes.
4. Stir in the flour and cook for an additional 2 minutes to remove the raw flour taste.
5. Slowly pour in the beef stock and Worcestershire sauce, stirring constantly to avoid lumps. Add the thyme and mushrooms, letting the mixture simmer until it thickens, about 10 minutes.
6. Return the beef to the skillet, coating it with the sauce. Let it simmer for another 5 minutes. Taste and adjust seasoning if necessary.
7. Transfer the beef mixture into the pie dish. Roll out the puff pastry and place it over the top, trimming any excess and crimping the edges to seal.
8. Brush the pastry with the beaten egg to give it a golden finish.
9. Bake in the preheated oven for 25-30 minutes, or until the pastry is puffed and golden brown. Let it cool for a few minutes before serving.

Having spent many Sundays experimenting in the kitchen, I’ve found that the key to a great pie lies in the balance of flavors and textures. Don’t rush the simmering process; it’s essential for coaxing out the rich, hearty flavors that make this dish a standout.