Whisk soy sauce, oyster sauce, rice vinegar, brown sugar, black pepper, broth, and sesame oil in a bowl.
Stir 0.5 tbsp cornstarch into the sauce until smooth; set aside.
Toss sliced chicken with 1 tbsp cornstarch and a small pinch of salt to lightly coat.
Heat oil in a large skillet over medium-high. Sear chicken in two batches until lightly browned, 3–4 minutes. Transfer to a plate.
Add mushrooms and onion to the skillet. Cook, stirring, until mushrooms are golden and onions soften, 4–5 minutes.
Stir in garlic and ginger for 30 seconds until fragrant.
Return chicken and any juices. Pour in the pepper sauce and simmer, stirring, until glossy and thickened, 1–2 minutes.
Fold in green onions. Taste and add more black pepper or salt if needed. Serve hot over rice.