Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the chicken pieces and season with salt and pepper. Cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.

In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced onion and bell pepper. Stir-fry for 2-3 minutes until they start to soften.

Add the sliced zucchini and minced garlic to the skillet. Stir-fry for another 3-4 minutes until the zucchini is tender but still crisp.

Return the cooked chicken to the skillet. Add the soy sauce, oyster sauce (if using), and sesame oil. Stir well to combine and heat through for another 2 minutes.

Taste and adjust seasoning with more salt and pepper if needed. Serve hot over cooked rice or quinoa.