Preheat the oven to 220°C, 425°F, Gas mark 7.
Heat the oil in a pan and saute the onion for 2 minutes before adding the beef. Cook for 7-8 minutes, stirring from time to time, to brown the meat off. Add the sliced mushrooms and continue to cook for another 5 minutes.
Spoon the beef mixture into a pie dish.
Roll out the pastry and use to cover the dish, pricking a hole in the middle to let out steam. Glaze the pie with a little milk and cook for 25 minutes, until golden. Serve hot, and enjoy.
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