Trim fat from meat. Cut 1/2-inch-wide slits about 1 inch deep into roast. Drizzle lemon juice over meats surface and into slits. Stir together parsley, mint, rosemary, onion salt, and pepper. Rub parsley mixture onto meat and into slits. Insert garlic slivers into slits .
Place meat, fat side up, on a rack in a shallow roasting pan. Insert an oven-going meat thermometer into center of roast . The thermometer should not touch the bone. Roast in a 325 degree F oven. For medium rare, roast for 1-3/4 to 2-1/4 hours until meat thermometer registers 140 degrees F. Cover with foil and let stand 15 minutes. The temperature of the meat after standing should be 145 degrees F. (For medium, roast for 2-1/4 to 2-3/4 hours or until meat thermometer registers 155 degrees F. Cover and let stand 15 minutes. The temperature of the meat after standing should be 160 degrees F.) If desired, serve with mint jelly.