Prep the chicken and veggies. Slice chicken into thin strips against the grain. Cut vegetables into bite-size pieces.
Keep everything ready near the stove; stir-fry moves fast.
Make the sauce. In a bowl, whisk soy sauce, water or broth, honey, vinegar, and sesame oil. In a separate small bowl, combine cornstarch with water to form a slurry. Keep both close.
Heat the pan properly. Set a large skillet or wok over medium-high to high heat.
Add 1 tablespoon oil and let it shimmer. A hot pan prevents sticking and keeps ingredients crisp.
Cook the chicken. Add the chicken in a single layer. Season lightly with salt and pepper.
Stir-fry 3–5 minutes until just cooked through and slightly browned. Remove to a plate.
Stir-fry the vegetables. If the pan is dry, add a splash of oil. Add firmer vegetables first (carrots, broccoli), cook 2 minutes, then add quicker-cooking ones (bell peppers, snap peas, mushrooms, onion).
Stir-fry until crisp-tender, 3–4 minutes.
Add aromatics. Push veggies to the sides. Add garlic and ginger to the center with a touch of oil. Cook 30 seconds until fragrant, then toss everything together.
Combine and sauce. Return the chicken to the pan.
Pour in the teriyaki base, stir, then drizzle in the cornstarch slurry. Toss until the sauce thickens and coats everything, 1–2 minutes. If too thick, add a splash of water; if too thin, simmer another minute.
Finish and serve. Taste and adjust with more soy (salt), honey (sweet), or vinegar (brightness).
Sprinkle sesame seeds and green onions if using. Serve over hot rice or noodles.