Slice the chicken breasts into thin strips. Slice the red bell pepper, green bell pepper, and onion. Mince the garlic and ginger.
In a small bowl, mix soy sauce, oyster sauce, and sesame oil together. Set aside.
Heat 1 tablespoon of vegetable oil in a large frying pan or wok over medium-high heat.
Add the sliced chicken to the pan. Season with a little salt and black pepper. Cook for about 5–6 minutes until the chicken is fully cooked and lightly golden. Remove the chicken from the pan and set aside.
Add the onion, garlic, and ginger. Stir for about 1 minute until fragrant.
Add the red bell pepper and green bell pepper. Stir-fry for 3–4 minutes until slightly tender but still crisp.
Pour the sauce mixture into the pan and stir everything together well.
Add the roasted cashews and mix gently.
Cook for another 1–2 minutes until everything is hot and coated with sauce.