Toss the diced beef steak in the plain flour until evenly coated.
Heat the vegetable oil in a large casserole dish over medium-high heat. Brown the beef in batches, ensuring it's well-seared on all sides. Remove and set aside.
In the same dish, add the chopped onion, diced carrots, and sliced mushrooms. Cook for 5-7 minutes until the vegetables are softened.
Add the minced garlic and cook for an additional 1-2 minutes until fragrant.
Return the beef to the dish. Pour in the dark ale and beef stock, then stir in the tomato purée, bay leaf, and fresh thyme. Bring to a simmer.
Cover and simmer the mixture on low heat for 1.5 to 2 hours, or until the beef is tender and the sauce has thickened. Remove the bay leaf.
Preheat the oven to 200°C (180°C fan)/Gas Mark 6. Roll out the puff pastry on a floured surface to fit your pie dish.
Spoon the beef mixture into the pie dish. Cover with the puff pastry, trimming any excess and crimping the edges to seal. Brush the top with beaten egg.
Bake in the oven for 25-30 minutes, or until the pastry is golden brown and crisp. Serve hot.