Preheat the oven to 350°.

Season the lamb all over with salt and pepper. In a very large, deep skillet or roasting pan, heat 2 tablespoons of the olive oil until shimmering. Add the lamb and sear over high heat, turning occasionally, until browned all over, about 12 minutes. Transfer to a plate.

In a small bowl, whisk the lemon zest with the garlic and remaining olive oil. Rub the mixture all over the lamb.

Arrange nine long pieces of kitchen string crosswise on a large rimmed baking sheet, 1 inch apart.

Arrange 10 rosemary sprigs across the string and set the lamb on top.

Cover with the remaining rosemary, then pull up each piece of string and tie tightly to secure the rosemary and form a neat roast.

Roast the lamb on the baking sheet for about 1 hour and 20 minutes, until an instant-read thermometer inserted in the thickest part of the meat registers 130°.

Transfer the lamb to a carving board and let rest for 30 minutes.