Prepare all ingredients: chop the chicken, dice the vegetables, and mince the garlic.
In a bowl, combine chicken with cornstarch, salt, and pepper. Mix well.
Heat 1 tablespoon of oil in a large skillet over medium-high heat.
Add the chicken and sauté until golden brown, about 5-7 minutes.
Remove chicken from the skillet and set aside.
In the same skillet, add the remaining oil and sauté the onion and garlic until fragrant.
Add the bell pepper and cook until tender, about 3-4 minutes.
Return the chicken to the skillet and stir in soy sauce and oyster sauce.
Add cashews and mix well to combine all ingredients.
Cook for an additional 2-3 minutes until everything is heated through.