How to Make It
Stir together cinnamon, red pepper, cumin, 2 tablespoons of the salt, and 1 teaspoon coriander in a small bowl. Rub spice mixture all over lamb. Drizzle with 3 tablespoons of the oil and rub to cover.
Preheat grill to high (450°F to 500°F). Grill lamb, covered, turning often, until seared on both sides, about 5 minutes. Reduce heat to medium (350°F to 400°F), and continue to grill until a thermometer inserted into thickest portion registers 120°F to 125°F, 15 to 20 minutes for medium-rare. Remove lamb from grill and let rest 10 minutes before slicing.
Meanwhile, make the chimichurri. Whisk together cilantro, parsley, vinegar, garlic, black pepper, remaining 3/4 cup oil, 1 teaspoon salt, and ½ teaspoon coriander in a small bowl.
Thinly slice lamb against the grain and spoon chimichurri over top.