Season chicken with salt, black pepper and red pepper flakes.
Preheat a wok over high heat.
Heat the olive oil in the preheated pan.
Add the chicken and stir-fry for 7 to 8 minutes, or until the chicken is cooked through and starts to brown.
In the meantime, whisk together the sesame oil, oyster sauce, soy sauce, chicken stock, cornstarch, garlic and rice vinegar.
Add the bell peppers and onion to the wok and cook for 3 minutes, or until the onion softens and becomes translucent.
Pour the prepared sauce over the chicken and cook for 3 more minutes, or until the sauce begins to thicken.
Add the cashews and green onions and toss everything together.
Remove from heat.
Serve over rice or noodles immediately.