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; 2 garlic cloves; 2 carrots; 2 celery stalks; 1 sprig fresh sage; 1 sprig fresh thyme; 40 g unsalted butter; 500 g stewing beef; 3 tbsp cornflour; 6 - 10 button mushrooms; 200 g Guinness®; salt; ground black pepper; 3 - 4 sheets puff pastry; 1 egg