Begin by placing sliced chicken breast into a mixing bowl. Pour in soy sauce, apple cider vinegar, and sugar, ensuring the chicken is evenly coated. Allow it to marinate for about 10 minutes.

In a separate bowl, whisk together the remaining soy sauce, apple cider vinegar, sugar, minced garlic, grated ginger, water, cornstarch, and red pepper flakes until smooth. Set it aside.

Heat a tablespoon of olive oil in a large wok or skillet. Add the marinated chicken in a single layer and sauté for 5-7 minutes until golden brown and fully cooked, then transfer to a plate.

In the same pan, add more olive oil if needed and toss in sliced onions. Cook for 3-4 minutes until caramelized and translucent. Then add diced zucchini, sautéing for an additional 3 minutes.

Pour the prepared sauce into the pan with the zucchini and onions, stirring constantly for about 2 minutes. Finally, return the cooked chicken to the pan, tossing together over medium heat for another minute until heated through.