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Heat the oil in a large saucepan. Crumble in the mince and stir to coat the meat in the hot oil. Cook the mince for 15 mins until you are left with browned, toasted mince and the fat from the meat. Stir the vegetables in with the mince and cook for 10 mins until they start to turn brown. Sprinkle over the flour and cook for a further 2 mins. Stir in the herbs, ketchup and tomato purée, and splash in the Worcestershire sauce. Pour in the ale, bubble for a minute, then gradually stir in the stock. Leave everything to simmer gently for 1 hr until you have a tender mince in a rich gravy.
Use a slotted spoon to lift the mince and vegetables into a large baking dish, leaving the residual gravy in the pan. Drain the gravy through a sieve into another saucepan, then tip what remains in the sieve back into the baking dish. Leave the mince to cool, then put in the fridge (or, better still, the freezer if you have room) for at least 1 hr until the fat from the mince solidifies. This part can be made up to two days ahead.
While the meat sauce is cooling, make the cheesy potato topping. Tip the potatoes, whole and in their skins, into a large pan of salted water. Bring the pan to the boil and simmer for 15 mins until the potatoes are cooked all the way through. Drain off the water and leave until the potatoes are just cool enough to handle. Meanwhile, boil the milk, butter and mustard powder together in a large saucepan. Peel the cooked potatoes by rubbing their skins off with a tea towel. Mash or pass the potatoes through a ricer into the hot milk and butter, stir through the cheese and season with salt and pepper.
When the meat sauce has set, top with the mash. Start from the outside and use a spatula to scrape the mash against the side of the dish, then spread it over the middle of the dish. Using a fork, make lines across the topping. Can be frozen for up to 6 months.
Heat oven to 190C/170C fan/gas 5. Put the pie on a large baking tray to catch any drips and bake for 30-35 mins until golden. Leave it to rest for at least 10 mins while you heat up the extra gravy. Serve the pie at the table with green vegetables and the jug of extra gravy for pouring over.