Combine cornstarch and pepper in resealable food storage bag. Add chicken. Shake to coat with cornstarch 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, sweet chili sauce, sugar, garlic, ginger, pepper flakes, and cashews in small bowl; pour over chicken. (I like my cashews to be softer so I add them during the cooking process, if you want more of a crunch, add them right before serving).