Prepare all your ingredients. Slice the chicken breasts, onions, and bell peppers, and mince the garlic and ginger.
In a mixing bowl, combine the sliced chicken with soy sauce, freshly ground black pepper, and salt. Allow it to marinate for about 10 minutes.
In a small bowl, mix cornstarch with water and sugar to form a slurry to thicken the sauce.
Heat the vegetable oil in a wok or large frying pan over medium-high heat.
Add the marinated chicken to the pan, spreading it out in a single layer. Cook for 3-5 minutes until the chicken is browned and cooked through. Remove the chicken from the pan and set aside.
In the same pan, add the onion, red bell pepper, and green bell pepper. Stir-fry for about 2-3 minutes until they're slightly softened.
Add the minced garlic and grated ginger to the vegetables and stir-fry for another minute until fragrant.
Return the cooked chicken to the pan and add the cornstarch slurry. Stir everything together until well combined and the sauce thickens, about 1-2 minutes.
Taste and adjust the seasoning if necessary with additional salt or black pepper.
Serve hot, garnished with chopped green onions, paired with rice or noodles.