Prep everything first. Slice the chicken thinly against the grain. Chop all vegetables and mince the garlic and ginger.
Stir together the soy sauce, water, rice vinegar, sweetener, mirin (if using), sesame oil, and red pepper flakes in a bowl. In a separate small cup, mix cornstarch with water.
Heat the pan properly. Set a large wok or wide skillet over medium-high to high heat. Add 1 tablespoon oil and let it shimmer.
A hot pan keeps the chicken searing instead of steaming.
Cook the chicken in batches. Add half the chicken in a single layer. Season lightly with salt and pepper. Sear 2–3 minutes per side until browned and just cooked through.
Transfer to a plate and repeat with the remaining chicken, adding a touch more oil if needed.
Stir fry the aromatics. Add a small splash of oil if the pan looks dry. Toss in garlic and ginger, stirring for 20–30 seconds until fragrant. Don’t let them burn.
Add the vegetables by density. Start with onions and carrots for 1–2 minutes.
Add broccoli for another 2 minutes, then bell pepper and snow peas for the final 1–2 minutes. You want crisp-tender, not soft.
Combine and sauce. Return the chicken to the pan. Give the sauce a quick stir and pour it in.
Once it starts to bubble, stir in the cornstarch slurry. Cook 1–2 minutes, tossing, until the sauce thickens and coats everything.
Finish and serve. Taste and adjust sweetness or salt. Remove from heat.
Garnish with sliced green onions and sesame seeds. Serve over rice or your favorite base.