Start by slicing the flank steak against the grain into thin strips. Place the beef in a bowl and add 3 tablespoons of soy sauce, 2 tablespoons of water, and 4 teaspoons of cornstarch. Stir well and set aside to marinate for 10-15 minutes.
While the beef is marinating, thinly slice the onion, bell peppers, mushrooms, zucchini, and carrot. Grate the fresh ginger.
In a large wok or frying pan, heat 2 tablespoons of vegetable oil over medium-high heat until shimmering.
Add the marinated beef in a single layer. Let it sear for about 2 minutes without stirring, then stir-fry for an additional 2-3 minutes until cooked through. Remove from the pan.
In the same pan, add the remaining tablespoon of oil. Add onion, bell peppers, and carrot, cooking for about 2-3 minutes. Then add mushrooms, zucchini, and grated ginger, cooking for another 3-4 minutes until tender-crisp.
Return the cooked beef to the pan, add the remaining 3 tablespoons of soy sauce and 1 teaspoon of sugar. Toss everything together and heat through for another minute.
Serve over cooked rice if desired, or enjoy on its own. Garnish with sesame seeds or green onions if you like!