Preheat the oven to 190°C/375°F/gas 5.
Trim any large bits of fat off the lamb, then cut the meat into chunks and put small batches into the food processor until minced roughly. Place the mince in a bowl, then add the flour and seasoning and toss until evenly coated.
Heat a large pan and, when it’s nice and hot, add the olive oil and lamb mince and fry until browned all over. Add the onion, celery, carrot, pancetta and garlic to the pan, and throw in a large pinch of rosemary leaves and the tomatoes. Pour in the stock and stir well so the mixture doesn’t stick to the bottom of the pan. Leave in the same pan or transfer to an ovenproof dish, cover and bake in the oven for an hour.
Meanwhile, peel the spuds, boil them in salted water until cooked through, then drain well. Heat the milk gently then pour over the potatoes. Add a knob of butter and mash well until smooth and creamy.
Melt the remaining butter in a frying pan. When it starts to bubble, throw in the rosemary and fry until crisp. Drain, and add the rosemary to the mashed potatoes with salt and freshly ground black pepper.
Take the lamb out of the oven, spoon over the mash, then turn up the temperature to 200°C/400°F/gas 6 and bake for about 20 minutes or until bubbling and crispy and brown on top.