Whisk together soy sauce, oyster sauce, black pepper, sugar, chicken broth, cornstarch, garlic, and ginger in a bowl and set aside.
Optionally toss chicken slices with cornstarch for tenderness.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add chicken and cook 4 to 5 minutes until lightly browned and cooked through. Remove and set aside.
In the same pan, sauté onion and mushrooms 3 to 4 minutes until mushrooms are tender and golden.
Return chicken to the pan and pour in the prepared sauce. Stir to combine.
Simmer 2 to 3 minutes until sauce thickens and coats the ingredients evenly.
Remove from heat, garnish with green onions and sesame seeds if desired, and serve hot with rice or noodles.