1. Preheat your oven to 180°C/Fan 160°C/Gas 4. Fill and boil your kettle. Peel the onions and thinly slice them. Trim and dice the carrot (no need to peel)s. Peel and thinly slice the garlic cloves. Trim the ends off the celery stick, and thinly slice it.
2. Pour 1 tbsp olive oil into a large ovenproof pan and warm over a medium-high heat. Tip the flour onto a plate and add the diced lamb with a good pinch of salt and pepper. Toss the lamb in the flour till it's completely coated. Add the lamb to the pan (save the plate of flour for later). Fry the lamb for 3-4 mins, turning once or twice, till well browned. Lift the lamb out of the pan and into a bowl.
3. Add the chopped onions, carrot, garlic and celery to the pan, with a splash of water if the pan seems a bit dry. Add the rosemary leaves and fry for 5 mins, stirring often, till the veg have browned a little. Tip in the reserved flour and stir for 2 mins.
4. While the veg fry, thinly slice the cavolo nero leaves.
5. Crumble the stock cube into the pan and add 1 tbsp Worcester sauce. Pour in 300ml hot water from the kettle. Tip the browned lamb back into the pan, and add the sliced cavolo nero. Stir well and bring to the boil, then turn down the heat so the pan is simmering.
6. Meanhwile, scrub the potatoes and slice them, as thinly as possible, into rounds. Take the lamb pan off the heat and spread the sliced potatoes, overlapping slightly, evenly over the surface. Brush the potatoes with olive oil and a pinch of salt and pepper. Slide the pan into the oven and bake for 30 mins, till the potatoes are tender and golden.
7. While the pie cooks, trim and halve the Brussels sprouts. When the pie is nearly ready, pour 1 tbsp olive oil into a large frying pan and warm to a medium heat. Add the Brussels sprouts with a pinch of salt and pepper. Fry for 4-5 mins, stirring occasionally, till the sprouts are tender. Squeeze in a little lemon juice.
8. Divide the potato and lamb pie between a couple of warm plates and serve with the lemony sprouts on the side.