Note the weight of the lamb and leave at room temperature, covered for 30 minutes. Pierce with a sharp knife and add slivers of garlic and rosemary sprigs. Brush with oil, season and place on an oven rack in a roasting dish.
Preheat the oven to 170ºC and roast the lamb for 15‐20 minutes per 500g (fan‐forced) or until cooked to your liking. Insert a skewer into the thickest part of the leg to the bone, and when the juices run clear the lamb is cooked.
Remove from the oven and rest 25 minutes in a warm place before carving.
To make gravy, drain all but 3 Tbsp juices from the roasting pan. Stir in 3 Tbsp flour, cook over a low heat for 2‐3 minutes, stirring well to remove lumps. Add 2 cups warm beef stock or water, scraping the bottom of the pan to incorporate the sediment, which is full of flavour. Simmer until thick and season to taste.