In a slow cooker, combine the chicken, bell peppers, onion, garlic, ginger, soy sauce, honey, rice vinegar, sesame oil, and red pepper flakes if using. Stir to coat the chicken evenly.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is tender and cooked through.
In a small bowl, mix the cornstarch and water until smooth. About 30 minutes before serving, stir the cornstarch mixture into the slow cooker to thicken the sauce.
Add the cashews to the slow cooker and stir to combine. Cook for an additional 30 minutes on high.
Serve the cashew chicken over cooked rice and garnish with chopped green onions.