DO-AHEAD TIP: Prepare brown rice.
In a small bowl or cup, combine soy sauce, water and cornstarch to a smooth paste then set aside. Heat the oil in a large skillet or wok over medium-high heat until hot.
Add garlic and half of the gingerroot and sauté for 1 minute, or until fragrant. Add all vegetables, except onion, and stir-fry for 2 to 3 minutes or until tender-crisp.
Stir in soy sauce mixture then onion, salt and the remaining gingerroot. Cook until mixture has thickened. Serve over cooked rice and enjoy!