1/2 cup packed fresh basil leaves
1/4 cup packed fresh mint leaves
1/4 cup packed fresh parsley leaves
2 T. toasted pine nuts
2 T. grated Parmigiano-Reggiano cheese
2 T. extra-virgin olive oil
1 clove garlic, peeled
1 tsp. salt, divided
1/2 tsp. freshly ground pepper
1 3 1/2 # boneless leg of lamb, butterflied and trimmed of fat
Preheat oven to 350
Place basil, mint, parsley, pine nuts, cheese, oil, garlic, 1/2 tsp. salt and pepper in food processor and process until fairly smooth. Sprinkle lamb all over with the remaining tsp. salt. Reserve 2 T. of the pesto; spread the rest over the top of the lamb and roll it closed. (It will not be a perfect cylinder.) Tie kitchen string around the roast in five places; do not tie too tightly or the pesto will squeeze out. Rub the reserved pesto over the outside of the lamb and place in a roasting pan.
Roast the lamb until a thermometer inserted into the thickest part registers 140 degrees F for medium-rare, about 1 hour 20 minutes. Transfer to a cutting board; let rest for 10 minutes (a lamb roast's internal temperature will rise about 10 degrees while resting). Carve the lamb, leaving the string in place to help hold the roast together.