Season the chicken with salt and a light dusting of black pepper. Heat oil in a large skillet or wok over medium-high heat. Add chicken in a single layer and sear for 3–4 minutes per side until golden and cooked through. Remove chicken and set aside.

In the same pan, add mushrooms and onions. Sauté 4–5 minutes until mushrooms are golden and onions begin to soften. Add garlic and sauté 30 seconds until fragrant.

Return chicken to the skillet. Sprinkle in freshly ground black pepper and stir to coat the chicken and vegetables.

Stir in soy sauce, oyster sauce (if using), sugar, and chicken broth, scraping up any browned bits from the pan. Bring to a simmer.

Stir in the cornstarch slurry, cook 1–2 minutes until the sauce thickens and becomes glossy.

Turn off heat. Drizzle with sesame oil if using and toss gently to combine.

Serve immediately, garnished with sliced green onions. Perfect over steamed rice, noodles, or alongside stir-fried veggies.