Cut chicken into large, 1½-inch pieces. Combine flour and pepper in ziplock bag, add chicken, and shake to coat with flour mixture.
Heat oil in large skillet over medium-high heat. Add chicken to skillet and brown chicken about 2 minutes on each side (note: do not cook chicken completely); transfer 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, if desired.