To make the sauce, whisk together soy sauce, ketchup, rice vinegar, brown sugar, garlic, ginger and red pepper flakes in a small bowl; set aside.
Heat olive oil in a large skillet over medium high heat.
Season chicken with salt and pepper, to taste. Add chicken to the skillet and cook, stirring occasionally, until browned, about 2 minutes.
Place chicken into a slow cooker. Stir in soy sauce mixture and gently toss to combine. Cover and cook on low heat for 2-3 hours. Stir in cashews and cook on low heat for an additional 15 minutes.
Serve immediately, garnished with green onions, if desired.