Start by cutting the chicken breast into 3/4-inch pieces. Wash and prepare the broccoli by cutting it into florets. Slice the onion into strips and thickly slice the mushrooms. Grate the ginger and garlic.
In a small bowl, season the chicken with black pepper and a tablespoon of soy sauce. Let it marinate for about 10 minutes.
In a large skillet or wok, heat 1 tablespoon of cooking oil over medium-high heat. Add the marinated chicken pieces and stir-fry for about 5-7 minutes until cooked through and lightly browned. Remove and set aside.
Add the remaining tablespoon of cooking oil to the skillet. Add the sliced onion, broccoli florets, and mushrooms. Stir-fry for about 5 minutes until tender yet crisp.
In a small bowl, mix the chicken broth, soy sauce, brown sugar, corn starch, sesame oil, ginger, and garlic. Stir until the cornstarch is dissolved.
Return the chicken to the skillet with the vegetables and pour the sauce over. Stir to coat evenly and cook for an additional 2-3 minutes until the sauce thickens.
Remove from heat and serve immediately over rice or noodles if desired.
