1. Chop the onions, carrots, and celery into small pieces.

2. Brown the beef cubes in a large pan over medium-high heat, then set aside.

3. Sauté the chopped vegetables in the same pan until softened.

4. Sprinkle flour over the vegetables, then gradually add Guinness stout and beef stock, stirring constantly.

5. Return the beef to the pan, add mushrooms, and simmer for 1 to 1.5 hours until tender.

6. Transfer filling to a pie dish, cover with puff pastry, and bake until golden and flaky.