Heat the wok or large frying pan over high heat and add the high-heat cooking oil.
Add the cubed, seasoned chicken and stir-fry until cooked through and lightly browned, about 4–5 minutes; remove if the pan becomes crowded or continue in place.
Add the chopped onion and cook until translucent, about 3–4 minutes.
Stir in the diced celery and carrots (and any other hard vegetables) and cook 3–4 minutes until they begin to soften.
Add softer vegetables (such as bell pepper, snow peas, zucchini, or mushrooms) and cook another 2–3 minutes so vegetables are tender but still slightly crisp.
Return the chicken to the pan if you removed it, pour in the soy sauce and sesame oil, and season with pepper to taste; stir to combine.
Add the cooked, cooled quinoa and toss thoroughly to combine and heat through, adjusting soy sauce or sesame oil if desired.
Serve immediately.