How to Make It
Set a wire rack inside a rimmed baking sheet lined with aluminum foil. Score the fatty top of the lamb 5 times (1/4 inch deep and 4 inches long). Stir together rosemary, thyme, 1/4 cup of the oil, and 1 tablespoon of the garlic. Rub mixture all over lamb. Season with pepper and 5 teaspoons of the salt. Place on wire rack on baking sheet. Let stand at room temperature 1 hour.
Preheat oven to 325°F. Cook lamb until a thermometer inserted in thickest portion of meat registers 125°F to 130°F for medium doneness, 1 hour, 45 minutes. Remove from oven; rest 30 minutes.
Meanwhile, increase oven temperature to 450°F. Return lamb to oven; cook until meat is browned and a crust has formed, 10 to 15 minutes. Rest on wire rack 15 minutes.
Process parsley, mint, vinegar, and remaining 1/2 teaspoon garlic in a food processor 20 seconds. With machine running, pour remaining 1 cup oil into mixture until blended, 20 seconds. Stir in remaining 1 teaspoon salt.
Slice lamb against the grain; serve with sauce.