Whisk the soy sauce, sesame oil, cornstarch, and water in a small bowl until smooth, then keep the sauce close to the stove.
Heat a large skillet or wok over high heat and add the vegetable oil once the surface becomes very hot.
Add the mushrooms and cook for 4–5 minutes, stirring occasionally, until they release moisture and begin browning around the edges.
Add the sliced zucchini, garlic, and ginger, then stir-fry for another 3–4 minutes until the zucchini is tender-crisp.
Pour the prepared sauce around the edge of the hot pan and toss immediately so it coats the vegetables evenly.
Continue cooking for about one minute until the sauce thickens slightly and becomes glossy.
Remove from the heat and serve immediately over steamed rice or noodles, adding sesame seeds or green onions if desired.