Marinate the chicken: In a bowl, combine the chicken pieces, soy sauce, and cornstarch. Mix well and let sit for 10-15 minutes while you prep the vegetables.
Heat the wok or skillet: Place a large wok or skillet over medium-high heat. Add 1 tablespoon of oil.
Cook the chicken: Once the oil is hot, add the marinated chicken pieces in a single layer. Let them cook undisturbed for a minute, then stir-fry until the chicken is cooked through and golden brown, about 4-5 minutes. Remove the chicken from the wok and set it aside.
Stir-fry the vegetables: Add another tablespoon of oil to the wok, then add the bell pepper, onion, broccoli, and carrot. Stir-fry for 4-5 minutes, or until the vegetables start to soften but are still crisp.
Add garlic and ginger: Add the minced garlic and ginger to the wok. Stir-fry for another 1-2 minutes until fragrant.
Combine chicken and vegetables: Return the chicken to the wok, then add the oyster sauce and rice vinegar. Stir everything together, ensuring the sauce evenly coats the chicken and vegetables.
Season and serve: Taste the stir-fry and adjust the seasoning with salt, pepper, or additional soy sauce if needed. Garnish with chopped green onions. Serve hot over rice or noodles.