Stir together all the sauce ingredients in a small bowl, making sure the cornstarch is dissolved well. Set aside.
Heat a wok or large skillet over medium-high to high heat. Swirl in 2 tablespoons of oil to the heated wok and coat the bottom. When you see wisps of white smoke, add the tofu, and stir-fry until light brown on the edges, about 2 minutes. Then add the ginger and stir-fry it until aromatic, about 30 seconds. Add all the remaining vegetables and keep them moving while searing. Don't be scared to scrape the pan and fold the vegetables over many times. Cook for about 1-2 minutes or until the Napa cabbage gets bright green and starts to soften.
Stir in the sauce, coat all the vegetables and tofu, and bring the sauce to a boil. The cornstarch will thicken into a glaze, about 1 minute. Remove from heat and serve immediately
PRO TIPS: Cut all the larger vegetables into 1/4-inch strips and keep smaller vegetables like snow peas and beansprouts whole.
The best tofu for the wok is tofu that is vacuum packaged, not the tofu in water. This tofu has naturally formed a skin and will not stick in the wok or pan. It comes naturally white, fried or often coated in a soy sauce or five-spiced glaze. It is usually in the deli section of a well-stocked grocery store and sometimes merchandised next to the cheese and cold cuts. Tofu in water should only be used for wet applications like soups or braises.
FOR VEGETARIAN: Vegetarian Oyster Sauce does exist! To make this dish legitimately vegetarian, use vegetarian oyster sauce. If you can't find it at your local market, Amazon does have it.