In a small bowl, whisk together the soy sauce, rice vinegar, chicken broth, and cornstarch until smooth. Set aside.
Heat olive oil in a large non-stick skillet over medium-high heat.
Add the sliced chicken breast to the skillet and cook for 4-5 minutes, stirring occasionally, until the chicken is cooked through and lightly browned. Remove chicken from skillet and set aside.
In the same skillet, add the sliced onion and cook for 2 minutes until slightly softened.
Add the minced garlic, grated ginger, and sliced mushrooms to the skillet. Cook for an additional 5 minutes, stirring occasionally, until mushrooms are tender and onions are translucent.
Return the cooked chicken to the skillet and pour the soy sauce mixture over the chicken and vegetables.
Stir well and cook for 2-3 minutes until the sauce thickens and everything is heated through.
Season with black pepper and red pepper flakes, stirring to combine.
Remove from heat and garnish with sliced green onions before serving.