Method

1. Preheat the oven to 350°F / 180°C.
2. Soak the rosemary branches in cold water for 1 hour. This will help prevent the rosemary from burning while they are in the oven.
3. Heat a heavy large cast -iron griddle pan over high heat.
4. Sprinkle the leg of lamb generously with salt and pepper.
5. Sear the leg of lamb until brown on all sides, about 5 minutes.
6. Transfer the lamb to a baking sheet.
7. Combine the olives, garlic, chopped rosemary, and pepper flakes in a Bump and Grind or another mortar and pestle.
8. Pound the olive mixture until the olives break apart. Add the olive oil and mix it into the olive mixture.
9. Lay the strands of kitchen twine horizontally across a large rimmed baking sheet, spacing them evenly apart.
10. Lay the rosemary branches perpendicularly atop the twine.
11. Stuff some of the olive mixture into the lamb cavity.
12. Spread the remaining olive mixture over the lamb.
13. Starting with the center twine and working toward the outer twines, tie the twine as tight as possible around the lamb and rosemary, completely encasing the lamb with the rosemary.
14. Arrange the lamb at an angle on the baking sheet.
15. Place the lamb in the oven.
16. Roast the lamb until an instant-read meat thermometer registers 125°F / 50°C when inserted into the thickest part of the lamb meat, turning the lamb occasionally, about 1 hour and 30 minutes.
17. Transfer the lamb to a carving board and let rest for 30 minutes.
18. Remove the twine and rosemary from the lamb.
19. Carve the lamb into slices and transfer to a platter and serve with any accumulated juices.