Place lamb, onions, garlic, coriander, cardamom, sweet potato and stock in a 5-litre capacity slow cooker. Season well with salt and pepper. Cover and cook on a low setting for 7 hours. Make sure to split cardamom pods down centre (not all way through) with a large sharp knife. This will help cardamom flavour to infuse in casserole. Unfortunately some of the black seeds may come out during the cooking process, so be careful not to eat them. They aren’t at all harmful to your health, just a little full-on in flavour.