Prep first. Thinly slice the chicken and chop all veggies. Stir together the sauce ingredients in a small bowl.
Have everything ready—this moves fast.
Heat the pan. Set a large skillet or wok over high heat. Add 1 tablespoon oil and let it shimmer.
Sear the chicken. Add the chicken in a single layer. Season lightly with salt and pepper.
Stir-fry 3–4 minutes until mostly cooked and lightly browned. Transfer to a plate.
Sauté aromatics. Add the remaining 1 tablespoon oil. Toss in garlic, ginger, and the white parts of the green onion.
Stir 20–30 seconds until fragrant.
Cook the veggies. Add broccoli, bell pepper, zucchini, and mushrooms. Stir-fry 2–3 minutes. You want crisp-tender, not soft.
Combine and sauce. Return chicken to the pan.
Pour in the sauce and toss to coat. Let it bubble 30–60 seconds. If using xanthan gum, sprinkle lightly while tossing to avoid clumps.
Finish. Stir in sesame oil (if not already in the sauce), red pepper flakes, and the green onion tops.
Taste and adjust salt, sweetener, or heat.
Serve. Top with sesame seeds. Enjoy as-is or over warm cauliflower rice or zoodles.