Steak and Mushroom pie

465g Diced casserole Beef
2 pinches dry mustard powder
1 tbsp cornflour
1 small onion,finely chopped
spring of thyme
1 Beef stock cube
1 tbsp tomato puree
250g chestnut mushrooms,halved
4 sheets chilled filo pastry
1 tbsp mild olive oil

Preheat oven to 160c/140c fan/gas 3.Put the beef in a casserole dish.Mix the mustard with the cornflour and season with pepper.Sprinkle onto the meat and toss until evenly coated.Add the onion and thyme.Dissolve the stock cube and tomato puree in 250ml hot water and to the dish.Cover with a lid and cook in oven for 2 hours30 mins.Add mushrooms to the dish.Cover again and return to oven for another 30 mins.Transfer to a pie dish.Increase the oven temp to 220c/200c fan/gas 7.Lay the sheets of filo pastry on a work surface and brush lightly with oil.Scrunch up the pastry loosely and put on the meat,to cover it.Bake for 15 mins,without the lid,until crisp and golden.Serve.