In a medium bowl, combine honey, soy sauce, tomato paste, apple cider vinegar, garlic, black pepper and red pepper flakes. Whisk until well combined.
Place the chicken in the slow cooker. Pour the sauce over the chicken.
Cover and cook on low for 4-5 hours or high for 2-3 hours, or until chicken is cooked through to at least 165° F and tender. If using boneless chicken thighs, cook on low for 4-6 hours or high for 2-4 hours.
Remove the chicken to a clean bowl or cutting board and let it rest for a few minutes.
Meanwhile, in a small bowl, stir together the 1 tablespoon cornstarch and 1 tablespoon water. Stir this cornstarch slurry into the sauce in the slow cooker. Cover and cook on high for 10-20 minutes, until the sauce thickens slightly.
Shred the chicken and return it to the sauce in the slow cooker.
Serve chicken and sauce over rice, garnished with chopped green onions or cilantro.