Pulse onion, parsley, garlic and spices in a food processor to form a coarse paste. Add lemon juice and oil; process until smooth.
Place lamb in a large, shallow pan. Pour marinade over lamb and turn to coat several times. Cover with plastic wrap and refrigerate overnight.
Position rack in bottom third of oven and preheat oven to 450°F. Remove lamb from marinade; wipe off excess. Place lamb on a rack in a shallow roasting pan and roast for 20 minutes; lower oven temperature to 325°F. Roast lamb until an instant-read thermometer inserted into thickest part of meat registers 140°F (for medium-rare), 1 hour and 15 minutes.
Remove meat to a cutting board; let rest for 20 minutes. Carve meat with tip of lamb bone facing you. Slice away from you and down, holding knife parallel to bone and turning lamb as you carve. Serve with Mint Pesto.