Preheat your oven to the highest heat (200-250°C).
Add the onion, garlic, and rosemary to the tray and sit the lamb on top (you don't want the lamb touching the base of the tray).
Drizzle olive oil over the top of the lamb, followed by a generous seasoning of salt and pepper.
Add enough stock to just touch the base of the lamb (you might not need the whole litre. If you need more, just top it up with water).
Place the tray in the oven for 20-30 minutes or until the lamb is golden on top.
Remove the tray from the oven and cover completely with foil.
Drop the oven temperature to 150°C and place the lamb back in for around 5 hours, checking it periodically to make sure the stock hasn't completely evaporated away. If so, just top it up with a little more.
You can alter the cook times if you'd like to cook it slower or quicker. If you bump the heat up to 180°C, you can get it done in around 3 hours, or if you'd prefer to do it overnight (8-12 hours), you can reduce the heat to 100°C.
