Healthy steak pie

Name: Healthy steak pie
Ingredients: 320g/11½oz sheet ready-rolled puff pastry (you will only use half); 1 tsp milk; 3 tsp sunflower oil; 250g/9oz thin frying steaks, cut into roughly 2cm/¾in pieces; 1 onion, finely chopped; 500g/1lb 2oz chestnut mushrooms, halved or quartered if large; 4 tsp plain flour; 1 tsp garlic powder (or 1 garlic clove, crushed); 350ml/12fl oz beef stock (made with 1 beef stock cube); 1 tbsp tomato purée; 1 tsp Worcestershire sauce; 1 tsp dried mixed herbs; salt and freshly ground black pepper