In a large, deep pot, heat a splash of oil over medium heat. Add the cubed beef steak, allowing it to sear for about 5-7 minutes until it's browned on all sides.

Once the beef is nicely browned, add the chopped onions, carrots, and celery to the pot. Sauté for approximately 5 minutes until the vegetables soften.

Sprinkle the plain flour over the meat and vegetables in the pot. Stir everything together and cook for an additional 1-2 minutes.

Pour in the Guinness stout, beef stock, and Worcestershire sauce, bringing the mixture to a gentle simmer.

Stir in the sliced mushrooms and let everything simmer on low heat for 15-20 minutes.

Once thickened, carefully transfer the hearty filling into a pie dish, spreading it evenly. Allow it to cool for about 10 minutes.

On a lightly floured surface, roll out the thawed puff pastry to cover your pie dish.

Drape the rolled pastry over the filling, pressing the edges firmly to seal. Cut a few slits into the top for steam vents.

Preheat your oven to 200°C (400°F) and bake the pie for 25-30 minutes until the pastry is beautifully golden brown.

Once baked, remove your Steak And Guinness Pie from the oven, and allow it to cool slightly before serving.