Heat the oil in a large frying pan over a medium heat and cook the onions and carrots for 5 mins, then add the potatoes and cook for a further 5 mins.
Gently stir through the corned beef and cook for 2-3 mins until beginning to colour. Add the Worcestershire sauce, the stock and some seasoning, then simmer for 5 mins or so until the potato is just tender.
Heat the oven to 200C/180C fan/gas 6. Transfer the filling to a pie dish or small ovenproof dish. If needed, gently roll your pastry to size, ensuring it covers the top of your tin or dish with a little excess.
Dampen the edges of the dish with water and lay the pastry on top. Use a fork to press the pastry firmly onto the edge. Make a small slash in the middle to allow steam to escape and trim off any excess pastry.
Put the pie on a baking tray, brush the pastry with milk and bake for 30-35 mins until golden brown. Serve with buttered kale, if you like.