As value wars take hold across fast food, Shake Shack aims to offer premium items at a discount.Building off its recent success with the $10 Dubai Chocolate Pistachio Shake, Shake Shack will launch its latest menu innovation Tuesday, this time featuring French onion flavors. The burger chain will introduce its French Onion Menu, featuring its new French Onion Soup Burger, first on its app on Sept. 9 and then across all channels on Sept.12. The burger is a made-to-order quarter-pound beef patty topped with G ...