In a large bowl, mix together the ingredients for the paste until smooth.
Prepare the lamb fillet by removing any sinew and cutting into 3 - 4cm (1 - 1.5 inch) chunks. Add the lamb to the paste and stir to coat thoroughly. Leave for a minimum of 30 minutes, but ideally 2 hours (or overnight if preparing the night before).
Sear the marinted meat in a pan, along with the onions. Transfer to the slow cooker.
In another bowl, mix together the yogurt, coriander, sugar and chopped chilli and then gradually stir in the passata and water. This will ensure a smooth consistency.
Pour the sauce over the lamb and onions in the slow cooker, then add the bay, cardamon pods and cinnamon stick and ensure they are submerged in the sauce. Cook on low for 6 hours or high for 3.5 hours.
Don't cut the pieces of lamb too small, 3-4 cm cubes or strips are ideal. The lamb becomes very soft and tender and this will prevent it falling apart in the sauce.
Fresh Chilli (and alternatives)
Taste a little bit of your red chilli before you dice it to check how fiery it is. If you want a hotter curry or the chilli proves to be quite mild then you may wish to add some or all of the seeds.Alternatively, use half a teaspoon of fresh chilli flakes.
Slow cookers vary in heat across makes and models. For reference, this recipe was cooked in a Crockpot CSC026 (linked in the equipment list).If your slow cooker has a tendency to cook quickly, sprinkle a tablespoon of cornflour into the slow cooker after adding the onions. This will prevent the natural yogurt splitting whilst cooking.
Can I freeze leftovers?
Absolutely! You can cool and freeze this straightaway and it will keep well for up to three months. It will also keep well in an air tight container in the fridge for up to three days.Defrost overnight in the fridge, and ensure it's piping hot all the way through before serving.
Can I make it in advance?
Yes. Cool straight after cooking and treat as if you were freezing leftovers. We eat most of the curries we make at a later stage!