How to offset sweetness in a sauce?
American versions of Mongolian BBQ are often pretty sweet (this doesn't sound like an authentic recipe). More soy sauce (or more of everything but the sugar) would cut the sweetness. Chili oil would certainly give it heat, but isn't going to temper the sweetness much. Any of the following would offset the sweetness and be appropriate for this type of dish:


Any tart apple (a common ingredient in the Japanese version of Mongolian BBQ)
Onions (not caramelized)
Rice wine vinegar