1. Slow Cooker: Place a large pan filled with water on a medium heat to boil. Meanwhile, peel and chop the potatoes and sweet potatoes into equal-sized chunks. Carefully add them to the simmering water and cook for 15-20 mins till the potatoes are tender when pierced with a knife.
2. Meanwhile, place a frying pan on a medium heat for 2 mins. Add 1 tbsp oil and add the lamb. Break the mince up with a wooden spoon and cook, stirring often, for 10 mins till browned. Tip the lamb into your slow cooker.
3. While the lamb is browning, put the tomato sauce, 1 tbsp Worcester sauce and 1 tbsp flour in a small bowl. Add a pinch of salt and pepper and mix till really smooth. Finely chop the rosemary leaves.
4. Peel and finely chop the onion. Trim, peel and dice the carrots. Trim the celery sticks and finely chop them. Add the veg and rosemary to the slow cooker. Crumble in the stock cube. Stir in the tomato sauce and 200ml water.
5. Drain the cooked potatoes. Add the butter and some salt and pepper. Mash till smooth.
6. Carefully spoon the mash over the lamb. Make a few holes in the mash with the back of a wooden spoon. Cook on a medium heat for 6-7 hrs. For a faster pie, set the slow cooker to high and cook for 5-6 hrs, or set to low and cook for 8-10 hrs. See our tip at the end about grilling the top.
7. When the pie has 15 mins left to cook, trim any thick stems from the kale. Finely shred the leaves. Zest and juice the lemon. Peel and grate the garlic. Place a frying pan on a medium heat. Add 1 tbsp oil, the kale and stir fry for 2 mins. Add the garlic, lemon zest and juice. Cook, stirring, for 2 mins. Serve the pie with the lemony kale on the side.
8. On the hob:
Place a casserole dish on medium-high heat for 2 mins. Add 1 tbsp oil and the lamb. Break up with a wooden spoon and fry, stirring often, for 10 mins.
9. While the lamb cooks, peel and finely chop the onion. Trim, peel and finely chop the carrot to match the onion. Trim and finely chop the celery.
10. When the lamb is browned, transfer with a slotted spoon to a bowl. Place the pan back on the heat and add all the chopped veg. Cook on a low heat, stirring regularly, for 15 mins till softened.
11. While the veg cooks, mix the tomato sauce, 1 tbsp Worcester sauce and 1 tbsp flour together with some salt and pepper. Chop the rosemary leaves.
12. After 15 mins, tip the lamb back into the pan. Stir in the thickened tomato sauce and the rosemary leaves. Cook, stirring, for 5 mins. Crumble in the stock cube and pour in 500ml boiling water. Pop a lid on the pan and cook for 1 hr, stirring every now and then.
13. While the lamb is cooking peel and chop the potatoes and sweet potatoes. Pop in a large pan and cover with cold water. Place on high heat and bring to the boil. Simmer for 20-25 mins till the potatoes are tender when pierced with a knife. Drain and tip back into the pan. Add the butter and some salt and pepper and mash till smooth and creamy. Pop a lid on to keep warm.
14. When the lamb has 15 mins left, trim any dry or really thick stems from the kale. Finely shred it. Zest and juice the lemon. Peel and grate the garlic.
15. Set your grill to medium-high. Taste the lamb and add some salt and pepper if it needs it. If your dish isn’t ovenproof, transfer the mix to a suitable one now. Spoon the potato mash over the top to cover the lamb mince. Slide the dish under the grill for 10 mins till the top is golden and slightly crispy.
16. While the pie is grilling, place a frying pan on a medium heat for2 mins. When warm, add 1 tbsp oil. Add the kale and stir fry for 2 mins. Add the garlic, lemon zest and juice with salt and pepper. Cook, stirring for 2 more mins. Serve the pie with the lemony kale on the side.