1. Preheat oven to 200°C (180°C fan-forced.
2. Place lamb on chopping board and season all sides with salt and pepper. Sprinkle garlic, rosemary and sage and massage into the meat.
3. Drizzle with olive oil on all sides and transfer meat into a roasting tray. Roast in the oven for 1 hour and 40 minutes.
4. In the meantime, make the crust. Heat oil in a large pan on medium heat. Add garlic and breadcrumbs and brown, stirring all the time. This will take about 2-3 minutes. Remove from heat and mix in the remaining ingredients.
5. Remove meat from oven tray and rest for 10 minutes.
6. To make jus, pour excess fat from the roasting tray, then heat tray on stovetop. Deglaze with red wine, stirring well to incorporate residue on tray bottom. Add stock and cook until reduced (by half). Whisk in cornflour and season with salt and pepper to taste.
7. Drizzle the jus over the lamb then sprinkle the citrus herb crust over the top. Slice thinly and serve.