Guinness beef pies

Name: Guinness beef pies
Prep time: 15 min
Cook time: 2 h 15 min
Total time: 2 h 30 min
Cuisine: European, British
Ingredients: 1 brown onion, cut into quarters; 2 garlic cloves; 2 carrots, cut into pieces; 2 celery stalks, cut into pieces; 1 sprig fresh sage, leaves only; 1 sprig fresh thyme, leaves only; 40 g unsalted butter; 500 g stewing beef (gravy beef or chuck steak), cut into pieces (3 cm); 3 tbsp cornflour; 6 - 10 button mushrooms, cut into quarters; 100 g bacon, cut into pieces (2 cm) (optional); 200 g Guinness®; 1 tbsp fish sauce (optional); salt, to taste; ground black pepper, to taste; 3 - 4 sheets puff pastry (see Tips); 1 egg, lightly beaten