MAKE THE SAUCE: In a small bowl, whisk together all the ingredients for the sauce.
In a large bowl, season chicken with salt and pepper and drizzle 1-2 tablespoons of sauce over the chicken. Allow to marinate for 30 minutes.
Heat cooking oil in a large skillet on medium-high heat. Add chicken and cook until brown, about 3-4 minutes. Pour in the remainder of the sauce and turn the heat to high. Allow sauce to bubble and thicken while stirring - about 1-2 minutes - add water as needed to thin out sauce as desired. Season to taste with salt, black pepper and/or red chili flakes.
Add zucchini noodles into the pan and use tongs to toss and coat with sauce. Cook for 1-2 minutes until zucchini is just tender but still firm. Be careful not to overcook.
Remove from heat and drizzle extra sauce in pan over chicken and serve immediately. Garnish with green onions and sesame seeds if desired.