Place the chicken breasts in the bottom of the Crockpot.
In a bowl, combine pineapple chunks (with juice), soy sauce, honey, ground ginger, garlic, apple cider vinegar, and red pepper flakes. Mix well.
Pour the sauce evenly over the chicken breasts in the Crockpot.
Cover and cook on low for 4-6 hours or on high for 2-3 hours.
Once cooked, remove the chicken and set aside.
In a small bowl, mix cornstarch and water to create a slurry. Add the slurry to the sauce in the Crockpot and cook on high for 10-15 minutes until the sauce thickens.
Shred the chicken with two forks and return it to the Crockpot, mixing it with the thickened sauce.
Garnish with fresh cilantro or green onions and serve.