Prep the chicken: Thinly slice the chicken against the grain. Pat dry with paper towels and season lightly with salt and black pepper.
Make the sauce: In a small bowl, whisk soy sauce, water, rice vinegar, honey, sesame oil, cornstarch, and red pepper flakes if using.
Set aside.
Preheat the pan: Heat a large skillet or wok over medium-high until hot. Add 1 tablespoon oil and swirl to coat.
Sear the chicken: Add half the chicken in a single layer. Cook 2–3 minutes per side until mostly cooked and lightly browned.
Transfer to a plate. Repeat with remaining chicken, adding a bit more oil if needed.
Cook firm veggies first: Add 1 tablespoon oil to the pan. Toss in broccoli and carrots with a pinch of salt.
Stir-fry 2–3 minutes until they start to brighten and soften slightly.
Add quicker-cooking veggies: Add onion, bell pepper, and snap peas. Stir-fry 2–3 minutes until crisp-tender.
Add aromatics: Push veggies to the sides. Add a small splash of oil if the pan is dry.
Add garlic and ginger to the center and cook 20–30 seconds until fragrant.
Combine and sauce: Return chicken and any juices to the pan. Whisk the sauce again and pour it in. Toss everything for 1–2 minutes as the sauce thickens and coats the stir fry.
Finish and serve: Taste and adjust with a splash more soy sauce or vinegar if needed.
Garnish with green onions and sesame seeds. Serve over rice, quinoa, or cauliflower rice.