In a large mixing bowl, combine soy sauce, teriyaki sauce, sesame oil, cooking oil, and minced garlic. Stir well.
Add diced chicken and mix until coated. Let marinate for 20–30 minutes.
Heat a cast iron skillet over medium-high heat until very hot.
Add the marinated chicken (in batches if needed to avoid crowding) and sear until golden brown and cooked through, about 6–8 minutes total.
Remove chicken from the skillet.
In the same skillet, add the sliced onion. Sauté until lightly browned, about 3–4 minutes.
Then add the diced zucchini and cook for another 3–4 minutes, until tender but not mushy.
Return the cooked chicken to the skillet and toss everything together.
Serve hot over cooked rice, noodles, or as-is.