Preheat the oven to 220C/200C fan/ Gas 7. Grease a 20cm square ovenproof dish (more deviation from me!). Heat the oil in a saucepan over a high heat. Saute the onion for 3 minutes or until softened. Add the mince and cook for 3 minutes or until browned.
Add the vegetables, gravy powder and 125mil hot water. (If using flour add a tbsp of flour and cook, stirring for 3 minutes to cook out the starch, crumble in the stock cube then add the water.) Cook stirring for 3 minutes or until the sauce is thick. Transfer the mixture to the prepared dish and set aside to cool for at least 5 minutes but longer if you can.
Place the pastry sheet over the mince to cover. Tuck the overhanging pastry inwards to form a border around the inside edge of the dish. Score the top of the pastry with a sharp knife. Brush with milk. Bake for 15 minutes or until browned.
Serve the pie with herbed mash or vegetables. I served my pie with slow-cooked Braised Red Cabbage.