In a large bowl, combine the chicken pieces with salt, black pepper, and cornstarch. Toss until the chicken is evenly coated.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the chicken and cook for about 5-7 minutes, stirring frequently, until the chicken is browned and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the sliced mushrooms and bell pepper. Stir-fry for about 3-4 minutes until the vegetables are tender.
Add the minced garlic and ginger to the skillet, cooking for an additional 1 minute until fragrant.
Return the chicken to the skillet. Pour in the soy sauce, oyster sauce, rice vinegar, and sesame oil. Stir well to combine and cook for another 2-3 minutes until everything is heated through and well-coated in the sauce.
Remove from heat and garnish with chopped green onions before serving.