Prep the chicken: Pat chicken dry with paper towels. Slice thinly across the grain for tenderness. Season lightly with black pepper.
Make the sauce: In a bowl, whisk soy sauce, chicken stock, oyster sauce, rice vinegar, sesame oil, honey, and cornstarch until smooth.
Set aside.
Heat the pan: Set a large skillet or wok over medium-high heat. 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 just cooked through, then transfer to a plate.
Repeat with remaining chicken, adding a splash more oil if needed. Do not overcook.
Stir-fry the veggies: Add remaining 1 tablespoon oil to the pan. Toss in broccoli and bell pepper.
Stir-fry 3–4 minutes until broccoli turns bright green and is crisp-tender. If needed, add 1–2 tablespoons water and cover for 30 seconds to steam.
Add aromatics: Push veggies to the sides. Add garlic, ginger, and red pepper flakes to the center.
Stir 30 seconds until fragrant.
Combine and sauce: Return chicken and any juices to the pan. Whisk the sauce again and pour it in. Toss for 1–2 minutes until the sauce thickens and coats everything.
Taste and adjust: Add a pinch more pepper, a splash of soy for salt, or a squeeze of lime for brightness.
Remove from heat.
Serve: Spoon over your favorite base. Garnish with green onions and sesame seeds. Serve hot.