Slice the boneless chicken breasts into thin strips. Season with a little salt and black pepper.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced chicken and cook for 5–7 minutes, stirring often, until the chicken is fully cooked and lightly browned.
Add the grated fresh ginger and minced garlic. Stir and cook for about 1 minute until fragrant.
Add the sliced onion and sliced bell pepper. Stir everything together and cook for 3–4 minutes until the vegetables are slightly tender but still crisp.
Pour in the soy sauce and oyster sauce. If using, add the chili flakes. Stir well to coat the chicken and vegetables evenly.
Add the cornstarch mixed with water. Stir continuously until the sauce thickens slightly and coats everything evenly.
Taste and season with more salt and black pepper if needed. Mix well.
Sprinkle the chopped spring onions over the stir fry and give everything a final stir.