SONIC introduces guilt-free SONIC Signature Slingers cheeseburger America's Drive-In® introduces the first QSR mushroom-blended burger starting under 350 calories OKLAHOMA CITY, 2018-Mar-07 — /EPR Retail News/ — SONIC® Drive-In (NASDAQ: SONC) revamped the traditional American cheeseburger with its new SONIC Signature Slingers, a first of its kind in fast food. The perfect-sized cheeseburger patty is made with three…