Heat 1 tbsp olive oil in a large saucepan and fry beef mince until browned – you may need to do this in batches. Set aside as it browns.
Put the olive oil into the pan, add finely chopped onions, chopped carrots and chopped celery sticks and cook on a gentle heat until soft, about 20 mins.
Fry the chopped Panchetta in a seperate pan until nice and crispy. Drain on a paper towel.
Add finely chopped garlic cloves, plain flour and tomato purée, increase the heat and cook for a few mins, then return the beef to the pan. Add the Panchetta
Pour over a large glass of red wine, if using, and boil to reduce it slightly before adding the beef stock, Worcestershire sauce, a few thyme sprigs and bay leaves.
Bring to a simmer and cook, uncovered, for 45 mins. By this time the gravy should be thick and coating the meat. Check after about 30 mins – if a lot of liquid remains, increase the heat slightly to reduce the gravy a little. Season well, then discard the bay leaves and thyme stalks.
Meanwhile, make the mash. In a large saucepan, cover the potatoes which you've peeled and chopped, in salted cold water, bring to the boil and simmer until tender.
Drain well, then allow to steam-dry for a few mins. Mash well with the milk, butter, and three-quarters of the strong cheddar cheese, then season with freshly grated nutmeg and some salt and pepper. Stir in the lemon zest.
Spoon the meat into 2 ovenproof dishes. Pipe or spoon on the mash to cover. Sprinkle on the remaining cheese.
If eating straight away, heat oven to 220C/200C fan/gas 7 and cook for 25-30 mins, or until the topping is golden.