“Frenched” leg of lamb means the meat, fat and membranes have been removed from the exposed bone. Ask the butcher to do it for you.
Let lamb stand at room temperature for 30 minutes. Trim fat from around hip (large) end. Trim excess fat from surface of lamb, leaving about 1⁄8 inch of fat. Make 1-inchwide- by-½-inch-deep slits over surface of lamb; insert 1 or 2 garlic slices into each slit. Rub lamb with 2 tablespoons of the oil; sprinkle with pepper and 2 teaspoons of the salt. Stir together parsley, oregano, minced garlic and 4 tablespoons of the remaining oil in a small bowl; rub 3 tablespoons of the mixture over lamb.
Preheat oven to 425°F. Place a rack in a large roasting pan. Stir pistachios, lemon zest and remaining 2 tablespoons oil and ¼ teaspoon salt into remaining parsley mixture. Place in a mini food processor and pulse until finely chopped, about 3 times. Reserve for serving with lamb.
Place lamb fat-side up on rack. Bake for 20 minutes. Reduce oven temperature to 325°F; bake lamb until a thermometer inserted into thickest part of flesh near the bone registers 130°F to 135°F for medium-rare, about 1 hour and 10 minutes. Transfer lamb to a cutting board; let stand for 10 to 20 minutes before slicing. Serve with reserved parsley mixture.