Heat three tablspoons of the sunflower oil in a large heavy-based frying pan and cook the onions gently for 20 minutes, or until softened and golden-brown.
Meanwhile place the cardamom pods in a pestle and mortar and pound lightly to split the pods. Tip onto a board and open each pod, scraping the seeds back into the mortar. Add the cumin, mustard seeds and cinnamon stick and grind into a fine, dry powder.
Remove half of the onions and transfer to a plate. Return the pan to the heat and stir in chillies and garlic and cook for three minutes. Add the ground spices, ground coriander and turmeric and fry, stirring constantly, for two minutes.
Preheat the oven to 170C/325F/Gas 3.
Add 300g/10z spinach to the pan and cook for 2-3 minutes. Remove from the heat, scrape into a heatproof bowl and set aside to cool.
Return the pan to the heat and add one tablespoon of oil. Season the mutton all over with salt and freshly ground black pepper. Fry the mutton over a medium-high heat in 2-3 batches until nicely browned on all sides, adding a little more oil if necessary. As soon as one batch is browned, transfer to a medium flame-proof casserole while the rest is fried.
Transfer the spiced onions and spinach to a food processor and blend to a thick green paste. Stir into the casserole dish with the lamb and add the bay leaf, tomato purée and 800ml/1⅓ pints cold water. Season with a teaspoon of sea salt flakes, stir well and bring to a simmer.
Remove from the heat and cover the surface of the curry with a piece of crumpled baking parchment. Place a lid on top of the casserole and cook in the oven for 2½-3 hours or until the mutton is very tender and the sauce is thick.
Stir in the reserved onions and the remaining spinach. Cover with the lid and return to the oven for a further 15-20 minutes or until the onions are hot and the spinach has wilted. Serve with rice or warm naan bread.