Prep the chicken: Thinly slice the chicken breast against the grain so it cooks fast and stays tender. Pat dry with paper towels.
Marinate briefly: In a bowl, mix soy sauce, cornstarch, sesame oil, and black pepper.
Toss with the chicken and let sit 10 minutes while you prepare the veggies and sauce.
Make the sauce: In a small bowl, whisk soy sauce, oyster sauce, rice vinegar, honey, chicken broth, cornstarch, and red pepper flakes if using. Set near the stove.
Pre-cook the broccoli: For fresh florets, microwave with a splash of water for 60–90 seconds or blanch in boiling water for 1 minute, then drain. For frozen, thaw and pat dry.
You want crisp-tender, not mushy.
Heat the pan: Set a large skillet or wok over medium-high heat. Add 1 tablespoon oil and swirl to coat until shimmering.
Sear the chicken: Add chicken in a single layer. Cook 3–4 minutes, flipping once, until just cooked through and lightly browned.
Do this in two batches if needed to avoid crowding. Transfer to a plate.
Sauté aromatics: Add the remaining 1 tablespoon oil. Toss in onion (if using) and cook 1–2 minutes until slightly softened.
Add garlic and ginger, stirring 30 seconds until fragrant.
Bring it together: Return chicken to the pan, add broccoli, and pour in the sauce. Toss constantly for 1–2 minutes as the sauce thickens and turns glossy, coating everything evenly.
Taste and adjust: Add a splash of soy for salt, vinegar for brightness, or honey for sweetness if needed. Finish with green onions.
Serve hot: Spoon over rice or your favorite base.
Garnish with sesame seeds and more green onions, if you like.