Slow cooker lamb rogan josh

Name: Slow cooker lamb rogan josh
Ingredients: 2 onions, 1½ roughly chopped, ½ thinly sliced; 100g/3½oz rogan josh spice paste (or follow pack guidance for serving 4 people); 600g/1lb 5oz lamb neck fillet or shoulder, excess fat trimmed and diced into 3–4cm/1¼–1½in chunks; 1½ tsp cumin seeds; 1 cinnamon stick; 2 fresh or dried bay leaves; 400g tin plum tomatoes; 1 pepper, any colour, cut into big chunks (optional); 100g/3½oz full-fat Greek-style or plain yoghurt; salt and freshly ground black pepper; freshly cooked rice (or options described in the introduction), to serve