Lamb:
20 large branches fresh long-stemmed rosemary
20 large branches fresh long-stemmed rosemary
One 6-pound semi-boneless leg of lamb (hip bone removed)
Kosher salt and freshly ground black pepper
1 cup pitted kalamata olives
6 garlic cloves, coarsely chopped
1 tablespoon chopped fresh rosemary
1 teaspoon dried red chile flakes
1/4 cup extra-virgin olive oil
Nine 24-inch-long strands of kitchen twine
The lamb can be assembled (not roasted) up to 1 day ahead, covered and refrigerated. Let the lamb stand at room temperature for 30 minutes before roasting.
Preheat the oven to 350°F.
Soak the rosemary branches in cold water for 1 hour. This will help prevent the rosemary from burning while they are in the oven.
Heat a heavy large cast-iron griddle pan over high heat. Sprinkle the leg of lamb generously with salt and pepper. Sear the leg of lamb until brown on all sides, about 5 minutes. Transfer the lamb to a baking sheet.
In a Bump and Grind or another mortar and pestle, combine the olives, garlic, chopped rosemary, and pepper flakes. Pound the olive mixture until the olives break apart. Mix in the olive oil.
Lay the strands of kitchen twine horizontally across a large rimmed baking sheet, spacing them evenly apart. Lay the rosemary branches perpendicularly atop the twine. Stuff some of the olive mixture into the lamb cavity and spread the remaining olive mixture over the lamb.
Arrange the lamb at an angle on the baking sheet so that it fits properly. Roast the lamb until an instant-read meat thermometer registers 125°F when inserted into the thickest part of the lamb meat, turning the lamb occasionally, about 1 hour and 30 minutes.
Transfer the lamb to a carving board and let rest for 30 minutes.
Remove the twine and rosemary from the lamb. Carve the lamb into slices and transfer to a platter and serve with any accumulated juices.