Prep

1. Take the lamb out of the fridge 30 minutes before cooking so it roasts evenly.
2. Heat the oven to 220°C (200°C fan, 425°F, gas 7). Put one shelf low down and one at the very top.
3. Pat the lamb dry with kitchen paper and rub it all over with the olive oil.
4. Cut 12–15 small slits, about 2 cm (¾ in) deep, all over the fatty side.
5. Push a sliver of garlic, a small sprig of rosemary and a cube of butter into each slit.
6. Season all over with the salt, pepper and coriander.

Brown

1. Spread the onions, bay leaves and thyme over the base of the roasting tin, then pour in the stock.
2. Sit the lamb on top of the onions, fat side up.
3. Pour the hot water into the small tray and put it on the top shelf.
4. Roast the lamb on the lower shelf for 15 minutes, until it starts to turn golden.

Slow roast

1. Turn the oven down to 180°C (160°C fan, 350°F, gas 4) without opening the door. Roast for 3 hours.
2. Every 30 minutes, turn the lamb over and baste it with the pan juices.
3. Add a splash of water if the tin looks dry, and top up the water tray as needed.
4. Cover loosely with foil for the last 30 minutes if the outside is getting too dark.
5. Check it’s done: a fork should slide in easily, and the meat should pull away from the bone.

Rest and serve

1. Tent the lamb loosely with foil and leave it to rest for 30 minutes.
2. While it rests, spoon the excess fat off the pan juices and mash the soft onions into them. Season to taste.
3. Carve into thin slices, or pull the meat apart with two forks. Serve with the onion gravy spooned over.