Place the chicken breasts in the bottom of the slow cooker.
Add the pineapple chunks, sliced red and green bell peppers, and sliced onion on top of the chicken.
In a medium bowl, whisk together pineapple juice, soy sauce, brown sugar, ketchup, apple cider vinegar, minced garlic, grated ginger, and crushed red pepper flakes until well combined.
Pour the sauce mixture evenly over the chicken and vegetables in the slow cooker.
Cover and cook on low for 4 to 4 1/2 hours, or until the chicken is cooked through and tender.
Remove the chicken and vegetables from the slow cooker and place on a serving platter, keeping warm.
In a small bowl, mix cornstarch and water until smooth to create a slurry.
Pour the liquid from the slow cooker into a saucepan and bring to a simmer over medium heat.
Stir in the cornstarch slurry and cook for 2-3 minutes, stirring constantly, until the sauce thickens.
Pour the thickened sauce over the chicken and vegetables.
Serve the Slow Cooker Chicken Hawaiian over cooked white rice.