Preheat the smoker to 275 degrees F (135 degrees C) for indirect smoking.
In a medium bowl, combine the oil, lemon juice, parsley, cilantro, oregano, garlic, jalapeno peppers, chili flakes, and salt to make the chimichurri sauce. Set aside.
Combine the olive oil, lemon juice, rosemary, and garlic in a bowl. Rub the lamb completely with the mixture to ensure that all the nooks and crannies are covered.
Use a sharp knife to make deep cuts, about 2 inches (5cm) apart, all over the lamb. Sprinkle all over with a medium coat of all-purpose rub.
If using a boneless leg of lamb, tie the lamb leg in to a log roll.
Smoke the lamb until the internal temperature is 110 degrees F (43 degrees C), 45 to 90 minutes depending on lamb size and your pit configuration. Once you reach the correct temperature, move the lamb over to direct heat and roast until the fat is rendered and the skin starts to brown and char.
Allow to rest for 10 minutes then cut into slices against the grain. Drizzle with the Chimichurri sauce and serve with additional sauce on the side.