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.

Heat olive oil in a large wok or skillet over medium-high heat. Add the marinated chicken in a single layer and sauté for 5-7 minutes until golden brown and cooked through.

In the same pan, add sliced onions and cook for about 3-4 minutes until caramelized. Then add diced zucchini, sautéing for an additional 3 minutes.

Pour the prepared sauce into the pan with the zucchini and onions, stirring constantly as it simmers for about 2 minutes. Then return the cooked chicken to the pan and toss everything together over medium heat for another minute.