Cut the lamb into large 4–5 cm chunks. Don’t cut it too small—you want substantial pieces after the long cook.

Mix everything together thoroughly and refrigerate for 3–4 hours. Even 2 hours will make a difference.

Take it out of the fridge about 30 minutes before cooking.

Heat the mustard oil in a heavy casserole until properly hot. If using mustard oil, let it just reach the point where it begins to smoke, then turn the heat down.

Add the 1 tsp sugar and let it caramelise until deep amber. Don’t let it actually burn.

Add the bay, cinnamon, cardamom, cloves, cumin seeds and dried chillies. Fry for about 30 seconds.

Add all the onions and a pinch of salt.

Now take your time: 20–30 minutes over medium/medium-low heat, stirring periodically, until they’re properly dark golden-brown. Don’t stop when they’re merely soft and translucent—the onion browning is fundamental to the finished curry.

Add the ginger and garlic and cook another 2 minutes.

Add cumin, coriander and chilli powder with a splash of water. Cook the spices for another 2–3 minutes, stirring, until the oil starts separating from the mixture.

Add the tomatoes and cook for 8–10 minutes, until they’ve almost disappeared into the masala.

Now add the lamb along with all its yoghurt marinade.

Turn the heat up to medium-high.

Cook for 15–20 minutes, stirring and turning the meat frequently.

This is the kosha/bhuno stage and it’s worth taking seriously. You’re effectively frying the lamb and its marinade into the onion-spice mixture. Initially it will release quite a lot of liquid; keep cooking until that has evaporated and the masala becomes thick, dark and glossy and starts clinging to the meat.

You’ll see little beads of fat/oil beginning to appear around the edges.

At that point you’ve built the flavour of the curry.

Add 250 ml hot water and scrape everything from the bottom of the casserole.

Bring just to a simmer, cover and put into a:

140°C fan / 160°C conventional oven

for approximately 2–2½ hours.

Check after about 90 minutes. Give it a stir and add another 50–150 ml water if it’s becoming too dry.

Because your lamb is chump rather than tougher traditional mutton, start checking tenderness at about 1 hour 45 minutes. You want it completely tender but still holding together.

This is what takes it from “nice lamb curry” to kosha.

Once the lamb is tender, put the casserole back on the hob uncovered over medium-low heat.

Cook for perhaps 10–20 minutes, stirring fairly regularly, until the sauce reduces dramatically.

You’re looking for very little free liquid. The gravy should be dark brown-red, glossy and thick enough that it coats each piece of lamb.

* 1 tsp garam masala

* salt to taste