Place the chicken breasts in the slow cooker.
In a medium bowl, whisk together honey, soy sauce, ketchup, minced garlic, ground ginger, and black pepper. Pour the mixture over the chicken.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is cooked through and tender.
Once cooked, remove the chicken from the slow cooker and set aside.
In a small bowl, mix the cornstarch and water to create a slurry. Stir this into the sauce in the slow cooker.
Turn the slow cooker to high and let the sauce thicken for about 10 minutes.
Return the chicken to the slow cooker, coating it with the thickened sauce.
Serve hot, garnished with sliced green onions.