1. Preheat your oven to 120°C/Fan 100°C/Gas ½. Peel and finely chop the onion. Place a shallow, ovenproof casserole dish on a medium-high heat and add 2 tbsp ghee or oil to the pan. Swirl round to coat the pan. Add the onion and season with a generous pinch of salt. Pop a lid on the pan and fry, stirring every few mins, for 10 mins till the onion is glossy and starting to soften and brown.
2. While the onion fries, dice the tomatoes. Trim and dice the aubergine. Peel and finely grate the garlic and ginger, or crush the garlic and finely chop the ginger. Finely chop 1-2 chillies, flicking out the white pith for less heat (you can use as much or as little chilli as you prefer).
3. When the onion has cooked for 10 mins, add the diced tomatoes to the pan and fry, stirring often, for 3-4 mins till the tomatoes are pulpy and have collapsed. Add the garlic, ginger and chillies and cook, stirring for 2 mins.
4. Add 1 tsp each ground cumin, coriander and turmeric. Drop in the cardamom pods and the cinnamon stick. Add the vegetable bouillon powder and demerara sugar. Stir really well to mix everything together. Add the tomato purée, dried red lentils and chopped aubergine. Stir well to mix. Pour in 850ml water and stir again. Place the lamb scrag on top of the veg and lentils.
5. Bring to the boil. When the water in the casserole dish is boiling, clamp on a lid and transfer the dish to the oven. Bake for 4 hrs, checking every 1-2 hrs. If it looks too dry, add more water and gently stir to mix, trying not to move the lamb too much.
6. After 4 hrs the lamb should be tender and fall off the bone if you press it with a fork. Take the dish out of the oven, lift the lamb onto a chopping board and use two forks to pull the meat off the bone. Chop or shred it, then stir it back into the lentils with the baby leaf spinach. Pop the lid back on the pan and let the curry sit for 2-3 mins. The heat will wilt the spinach. Stir in the wilted spinach.
7. Taste the curry and add 1-2 tbsp lime juice and a few pinches more sugar or salt, to taste. Serve the curry with your choice of rice or naan breads and yogurt, garnished with coriander leaves and thinly sliced red chilli.