Pat lamb dry and score fat by making shallow cuts all over with tip of a sharp small knife.
Place garlic, parsley, mint, lemon zest and juice, salt, pepper, and oil in a food processor and process until smooth. Rub the paste all over the lamb and place in a lightly oiled roasting pan. Let stand at room temperature for 30 minutes.
Preheat oven to 425 degrees F.
Roast lamb in middle of oven for 20 minutes, reduce heat to 350, and cook until an instant-read thermometer inserted 2 inches into thickest part of meat (not touching bone) registers 135 degrees F, about 1 3/4 hours.
Transfer to a cutting board and let stand 15 to 25 minutes (the internal temperature will rise to about 145 degrees F for medium). Slice the meat against the grain and serve with pita.