In a small bowl, wisk together soy sauce, brown sugar, cornstarch and water until well mixed; set aside.
Heat 1 Tbsp of oil in a large wok or fry pan over medium-high heat. Add onion, broccoli and water chestnuts. Stir fry them for 5 minutes. Remove from wok and set aside.
Heat the remaining 1 Tbsp of oil in wok. Add beef and garlic. Stir fry for 2 minutes or until beef in browned and no longer red.
Return broccoli/onions/water chestnuts to the wok. Stir in soy sauce mixture and cook until the sauce thickens (about 5 minutes).
Great served over brown or white rice.
Most soy sauce contains wheat/gluten. To avoid wheat/gluten, use a gluten-free soy sauce.
If avoiding soy, coconut aminos can be substituted for the soy sauce.