Add chicken and cornstarch to a large ziplock bag. Toss to coat the chicken with the cornstarch.
Heat oil in a large skillet or wok. Saute chicken until golden brown on both sides about 2 minutes on each side. (Note the chicken will not be fully cooked and that is ok) Add the chicken to the slow cooker.
Add hoisin sauce, vinegar, sugar and sesame oil to a medium bowl and whisk until combined. Pour the sauce onto the chicken and stir to coat the chicken.
Cook on low for 2-3 hours.Serve with green onions and sesame seeds for serving, if desired.