Heat the oil in a large saucepan over a medium heat and fry the whole spices for about 20 seconds until they start to sizzle. Add the onions and a pinch of salt, and fry gently until they are meltingly soft, deeply browned and rich, about 20 mins. Do not rush this part – this step is important, as the overall colour of the curry depends on it. Meanwhile, tip the chopped tomatoes into a food processor and blitz until smooth, or use a hand blender and a jug. Set aside.
Add the ginger and garlic pastes to the pan and cook for 30 seconds. Add the lamb, 2 tsp black pepper and the turmeric, and stir well to coat. Cover and cook over a medium heat for 15 mins, then remove the lid and cook for a further 15 mins until the liquid from the meat has evaporated and the oil comes to the top and side of the pan.
Add the ground coriander and cumin, both chilli powders, the tandoori masala, diced tomatoes, blitzed tomatoes and tomato purée, and cook until the tomatoes soften and dissolve, about 4-5 mins.
Season with salt and add 500ml water, depending on how thick you want the curry. Simmer until thickened and the meat is tender, about 1 hr 30 mins-1 hr 45 mins. If the sauce needs to be reduced slightly, simmer, uncovered, for a further 15 mins. Garnish with the coriander, sliced chillies and potatoes (see tip, below), and serve with rice or naan.
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