Place the chicken breasts into the slow cooker and lightly season both sides with salt and pepper. In a medium bowl, combine onion, garlic, honey, ketchup, soy sauce, oil, and red pepper flakes. Pour the sauce over the chicken in the slow cooker. Cover and cook on low for 3-4 hours, or on high for 2 hours until the chicken is fully cooked (the internal temperature has reached 165 degrees).
Once the chicken has finished cooking, remove the chicken breasts and place them onto a cutting board, leaving the sauce in the slow cooker. Shred the chicken into bite-sized pieces using two forks. Set aside.
In a small bowl, dissolve 4 teaspoons of cornstarch in 1/3 cup water, add to the crockpot. Stir to combine with the sauce. Cover and cook sauce on high for ten more minutes, or until the sauce has slightly thickened.
Return the chicken to the slow cooker and toss with the sauce. Serve over cooked rice if desired. Top with sesame seeds and sliced green onions.