Heat the slow cooker if you need to. Season the lamb, then brown it in batches for 5-8 mins using a little oil for each batch. Transfer each batch of lamb to the slow cooker.
Put the onions in a frying pan with a little more oil and fry, stirring, for around 10 mins until tender. Stir in the garlic, ginger, spices, chilli and tomato purée and fry together for 1 min more. Add everything to the slow cooker.
Pour 200ml boiling water into the slow cooker and mix everything well. Cook on a low heat for at least 6 hrs.
After at least 6 hrs of cooking, add the yogurt and stir well. Replace the lid and cook for a further ½ to 1 hrs on low. Don't worry if the sauce splits a little, just stir well before serving. Once cooked, scatter over the coriander and flaked almonds and serve with the rice.