In a small bowl, whisk together the oyster sauce, low sodium soy sauce, dark soy sauce, white vinegar, sugar, water, and cornstarch. Set this mixture aside; it will serve as the flavorful sauce for your dish.
In your wok or skillet, heat 1 tablespoon of peanut oil over medium heat. Add the raw cashews and toast them for about 3-4 minutes, or until they are golden brown. Remove the cashews from the pan and set them aside.
In the same pan, add another tablespoon of peanut oil. Once hot, add the sliced onion and minced garlic. Sauté for 2-3 minutes until the onion becomes translucent and fragrant.
Add the chicken thighs to the pan, and cook for 5-7 minutes, stirring occasionally, until the chicken is browned and cooked through.
Stir in the bell pepper and dried red chilies. Sauté for another 3-4 minutes until the bell pepper is tender but still crisp.
Pour the prepared sauce over the chicken and vegetables, stirring well to coat everything evenly. Allow the sauce to simmer for a few minutes until it thickens slightly.
Add the toasted cashews and green onions to the stir-fry. Toss everything together and cook for an additional minute to heat through.
Serve your Thai Cashew Chicken hot over steamed rice or noodles, and enjoy the delightful flavors and textures.