Mix cornstarch and soy sauce until there’s no lumps. Then add the mirin, oyster sauce, sesame oil, and pepper and mix to combine.
Place the chicken pieces in a bowl and add 2 tablespoons of the sauce mixture, toss to coat. Set aside to marinate for 30 minutes.
Heat the peanut oil over medium-high heat in a sauté pan or skillet. Add the garlic and onion, and cook for 1-2 minutes.
Add chicken and cook for 2 more minutes, making sure to get all sides. Add the bell pepper and cook for a few more minutes.
Add the rest of the sauce mixture and the water to the pan and bring to simmer. Cook, stirring occasionally, until the sauce thickens to the desired consistency.
Stir in the cashews and remove the pan from the heat. Serve immediately with rice, if desired.