Whisk 1/2 tbsp oil, the oyster sauce and cornstarch into a thick paste and add the chicken pieces. Stir until coated and set aside.
Prepare the sauce by stirring together the soy sauce, water, rice vinegar, sugar, sesame oil, red pepper flakes, ginger and garlic.
Add the remaining 1 tbsp of oil to a large skillet or frying pan and preheat over a high heat. Add the chicken and sauté until the pieces are browned and just cooked through. Transfer chicken to a dish and cover to keep warm.
Add the bell pepper, onions and cashews, and cook until the onions are just starting to soften and color.
Pour in the sauce and once it is bubbling return the chicken to the pan.
Reduce the heat to medium high and cook until the sauce has thickened to coat the chicken and vegetables. Serve with white or brown rice, topped with the sliced green onions.