Combine flour and pepper in re-sealable bag. Add chicken. Shake to coat with flour mixture. Heat oil in skillet over medium-high heat. Brown chicken about 2 minutes on each side. Place chicken in slow cooker.
Combine soy sauce, vinegar, ketchup, sugar, garlic, ginger, and pepper flakes in small bowl; pour over chicken. Cook on LOW for 3 to 4 hours. Add cashews and stir. Serve over rice and garnish with chopped green onions.