(Up to a month ahead) heat the oil in a large pan. Fry the bacon lardons over a high heat for several mins until well coloured. Using a slotted spoon, remove from the pan, drain on kitchen paper and set aside.
Season the beef, then fry in the fat left in the pan. Once coloured, add all the remaining ingredients for the beef, except the cornstarch and bacon, and bring to a simmer. Partly cover the pan with a lid and continue to cook for a further 2-2½ hrs or until the beef is tender.
Once cooked, tip everything into a colander set over another pan so you can catch the sauce. Tip the contents of the colander into a large pie dish, about 11in x 9in x 2in.
Re-boil the sauce and check for seasoning. Thicken with the loosened cornstarch, adding a little at a time and stirring until you have the desired coating consistency. Spoon enough sauce over the beef to barely cover and loosen it, then stir. Remaining sauce can be frozen, ready to re-heat and offer separately on the day.
While the beef is cooling, make the mash. Boil the potatoes until tender, 15-20 mins, depending on size. Drain off the water and replace the lid. Shake the pan to break up the boiled potatoes. Add butter and milk a little at a time while mashing. Season.
Spoon and spread the mashed potatoes on top of the cooled beef. Once covered, the mash can be marked with a spoon, palette knife or fork for a more interesting finish. When cold, the cottage pie is ready to cover and freeze.
The day before serving, transfer the pie to the fridge to defrost overnight. (Or reheat from frozen on the day, see below.)
On the day, reheat the pie. Heat oven to 350°F If thawed, bake for 50 mins-1 hr, or until heated through, then increase heat to 400F for the final 10 mins for a golden brown topping.
If cooking from frozen, cover with foil, then bake for 1¼ hrs; remove foil and bake uncovered for 1¼ hrs more, then increase heat as above for the last 10 minutes.