In a small bowl, combine all the ingredients except for the lamb.
Untie the lamb and rub about half of the marinade on the inside of it. Wrap the lamb back up and tie it tightly. Rub the remaining marinade on the outside and place in a non-reactive container. Cover tightly and refrigerate for about 24 hours, or at least overnight.
Once you’re ready to cook the lamb, preheat the oven to 275ºF and place the lamb in a roasting dish or baking sheet fit with a metal rack. Cook the lamb until the internal temperature reaches 125ºF-130ºF for medium rare, 130ºF-135ºF for medium. (see recipe notes) Remove the lamb from the oven and turn the temperature up to 500ºF. Allow the lamb to rest while the oven heats, or at least for 15 minutes, then return it to the oven and roast for 8-10 minutes, or until the exterior is nicely browned. (While the lamb rests and the oven heats to 500ºF is a great time to make the chimichurri)
Allow the lamb to rest for at least 10 minutes before you untie and slice it. Garnish with parsley leaves and mint leaves, and serve with the Parsley-Mint Chimichurri.
To make the chimichurri:
Place the parsley, mint, lemon juice, lemon zest, and sea salt in the bowl of a food processor. Pulse 10-12 times, or until the mixture is finely minced.
Turn on the motor and pour the olive oil through the chute. Process until incorporated.
Notes
Harissa can be found in jars in the ethnic section of Whole Foods Market
How long it takes to cook your lamb will depend on the weight and thickness of it. Mine was 3.5 pounds and took just under 2 hours. I recommend checking the temperature with an instant read thermometer after an hour to see where you’re at, and then checking it every 15 minutes or so.