Season both sides of the chicken with lightly with salt and pepper and place in the slow cooker.
In a small bowl, combine the honey, soy sauce, onion, tomato sauce, vinegar, oil, garlic and pepper flakes. Pour over the chicken. Cook on low for 3 to 4 hours or on high 1 1/2 to 2 1/2 hours, or just until chicken is cooked through and tender.
Remove the chicken from slow cooker, leaving the sauce. Dissolve the cornstarch in 1/3 cup of water and pour into the slow cooker. Add in the sesame oil. Stir to combine with the sauce. Replace the lid and cook sauce on high for ten more minutes or until slightly thickened. Shred the chicken into bite size pieces, then return to the pot and toss with the sauce before serving. Sprinkle with the sesame seeds and scallions and serve over rice or noodles.