Whisk together soy sauce, sugar, cornstarch and water in small bowl until blended; set aside.
Heat 1 tablespoon oil in large skillet or wok over medium-high heat. Add broccoli, water chestnuts and onion; cook and stir 4 minutes or until crisp-tender. Remove from skillet; set aside.
Place remaining 1 tablespoon oil in skillet. Add beef and garlic; cook and stir 2 minutes or until beef is browned. Return broccoli mixture to skillet. Stir in soy sauce mixture; cook until sauce thickens. Top with chow mein noodles, if desired.