Method

Preheat oven to 220°C fan
Rub the lamb with the olive oil, salt and pepper.
Use a thin, sharp knife to make 12 incisions in the lamb, deep as you can but without piercing through the bottom of the lamb. Stuff bits of rosemary and garlic slivers into the holes.
Place the onion, halved garlic bulb and rosemary in the base of a roasting pan. Place the lamb on top. Pour water around. Cover with lid or tightly with a double layer of foil. Place in the oven, and TURN DOWN to 160°C fan.
Roast, covered with the foil, for 3 hours. (for smaller or boneless pieces reduce cooking time by 20 mins).
Remove foil, check to ensure there's still liquid in the pan. If not, add 3/4 cup water (otherwise onion/garlic will burn). Turn up the oven to 220°C and roast for a further 20 to 30 minutes, until the skin is browned and crisp.
It should be possible to part the meat with two forks - if not, just cover and return to oven at 160°C fan until it is.
Remove the lamb from the roasting pan and transfer to a plate. Cover loosely with foil and let it rest for at least 20 minutes, up to a couple of hours (after this, you may want to reheat).
Serve with gravy (below).

GRAVY
Tilt the pan and use a spoon to remove all but around 2 tbsp of fat (try to avoid scooping out any juices).
Place the roasting pan on the stove over medium high heat. Add the flour and stir to mix in with the fat. Cook for 30 seconds.
Add the stock gradually and stir to combine. Use a potato masher to mash the onion and garlic, making sure that all the garlic squeezes out of the skin. Allow it to simmer for 1 to 2 minutes until it is just before your desired consistency (it will thicken a bit as it cools), then remove from the stove. Season to taste with salt and pepper, strain into a bowl being sure to squeeze all juices out of garlic etc, then transfer into gravy jug.