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

In the same pan, add onions. Cook for 3-4 minutes until caramelized. Then, add diced zucchini, sautéing for another 3 minutes.

Pour the prepared sauce into the pan with zucchini and onions, stirring as it simmers for about 2 minutes. Return the cooked chicken and toss everything together for another minute.