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-inch-wide-by-1/2-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 1/4 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.