Melt the butter, add the oil, then in go the mushrooms, spring onions and garlic.
Gently fry until the mushrooms are a golden brown colour.
Peel and chop the potatoes, and add them to the pan together with the frozen veggies and beef.
Pour in the stock, and leave to cook for 15 minutes or until the veggies are tender.
Mix the cornflour with milk to get a lump-free liquid, then add it to the filling to get a thicker sauce.
Season with salt and pepper, take it off the heat and leave to cool completely.
Transfer the filling to an oven-proof dish, and brush the edges with beaten egg.
Cut the puff pastry to make a lid for the dish - it all depends on the size of your dish.
Brush with more beaten egg, and use a sharp knife to make a few cuts in the pastry to allow the steam to escape.
Bake at 180 degrees Celsius (350 Fahrenheit) for 25 minutes until golden and flaky.