In a bowl, combine thinly sliced chicken with soy sauce, apple cider vinegar, and sugar. Let it marinate for about 10 minutes.

In a separate small bowl, whisk together remaining soy sauce, vinegar, sugar, ginger, minced garlic, water, cornstarch, and red pepper flakes.

Heat a large skillet or wok over medium-high heat and add olive oil. Cook the marinated chicken for about 5–7 minutes until golden brown.

In the same skillet, add sliced onions and sauté until translucent and caramelized for 4–5 minutes.

Add chopped zucchini and pour the prepared sauce over vegetables. Cook for about 2–3 minutes until zucchini is tender yet crisp.

Return the cooked chicken to the skillet, tossing everything together to coat. Stir-fry for another 2 minutes.

Transfer to serving dishes and garnish with sesame seeds and finely chopped green onions.