1- Preheat the oven to 240°C (465°F), or 220°C (425°F) if using a fan-forced oven.

2- Rub the lamb shoulder with olive oil, salt, and black pepper.

3- Using a sharp thin knife, make 12 deep incisions in the lamb without cutting through the bottom.

4- Stuff the rosemary sprigs and garlic slivers into the incisions.

5- Place the quartered onion, halved garlic bulb, and some rosemary sprigs in the base of a roasting pan.

6- Set the lamb shoulder on top and pour 1 cup of water around the base.

7- Cover the roasting pan with a lid or double-layered foil.

8- Reduce the oven temperature to 180°C (350°F) or 160°C (320°F) fan-forced, and roast covered for 3 hours.

9- Remove the foil, check if liquid remains; if dry, add 3/4 cup water to prevent burning.

10- Increase the oven temperature to 220°C (425°F) and roast uncovered for 20 30 minutes until the skin is browned and crisp.

11- Check the tenderness by pulling the meat apart with forks; if not tender, cover again and return to the oven at 180°C until it easily pulls apart.

12- Rest the lamb loosely covered with foil and towels for at least 20 minutes before serving, up to a couple of hours.

13-For the Gravy:

Remove all but 2 tablespoons of fat from the roasting pan.

Place the pan over medium-high heat, add 2 tablespoons flour, and cook for 30 seconds.

Gradually add 2 cups beef broth or a mixture of 1 cup red wine and 1 cup water, stirring constantly.

Mash the onion and garlic in the pan to extract juices.

Simmer the gravy for 1 to 2 minutes until thickened.

Season with salt and pepper, then strain before serving with the lamb.