Preheat your oven to 170°C/335°F (standard) or 150°C (fan).

In a metal roasting pan, place the garlic, onion, and rosemary.

Position the leg of lamb right side up in the pan. Generously sprinkle with salt and pepper and rub the seasoning into the meat. Turn the lamb over and ensure it rests mostly on the garlic and onion, repeating the seasoning.

Pour the olive oil over the lamb, then add the beef stock and water around it. Cover the roasting pan with foil, ensuring steam can escape.

Slide the lamb into the oven and roast it for about 4.5 hours.

After cooking time is up, remove the foil and test the lamb for tenderness using a fork. If not tender enough, cover again and return to the oven.

Leave the lamb uncovered for an additional 45 minutes or until a beautiful brown crust forms.

In a small saucepan, whisk flour with 1 cup of water until smooth. Place over medium heat.

Gradually add the pan juices, stirring constantly until it thickens and coats the back of a spoon. Season with salt and pepper to taste.

Once done, remove the lamb and spoon pan juices over it. Transfer to a serving platter and loosely cover with foil to keep warm while you prepare the gravy.