Marinate chicken: In medium bowl, stir together marinade ingredients until cornstarch dissolves. Add chicken; stir to coat. Let stand at room temperature for 10 to 15 minutes.
Prepare the sauce: In a small bowl, mix together soy sauce, rice wine, oyster sauce, sesame oil and chili sauce (if using). Set aside.
Heat a work or large skillet over medium-high heat until a bead of water sizzles and evaporates. Add oil; swirl to coat base. Add garlic and stir-fry until just aromatic, about 20 seconds. Add chicken and stir-fry for about 2 minutes, until no longer pink on the outside but not yet cooked through.
Add onions and bell peppers and stir-fry untill fragrant and the onions start to become translucent, 1 to 2 minutes.
Pour in the sauce and stir to coat the chicken. Let the sauce simmer for 2 minutes to thicken and allow the chicken to cook through, then stir in the cashews. Transfer to a plate and serve.