Heat 3 tbsp sunflower oil in a pan and cook onions for 20 minutes until golden.
Crush cardamom seeds, cumin, mustard seeds, and cinnamon into a fine powder.
Reserve half the onions, then fry chillies and garlic with the ground spices in the pan.
Preheat the oven to 170°C.
Cook 300g spinach in the pan for 3 minutes and set aside to cool.
Brown seasoned mutton in batches with the remaining oil and transfer to a casserole dish.
Blend the spiced onions and cooled spinach into a paste and add to the casserole.
Add bay leaf, tomato purée, 800ml water, and salt, then bring to a simmer.
Cover with parchment and a lid, then bake for 2.5 to 3 hours until tender.
Stir in reserved onions and remaining spinach, then bake for 20 more minutes.