Place the chicken breasts in the bottom of the slow cooker.
Add the pineapple chunks and sliced red and green bell peppers on top of the chicken.
In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, rice vinegar, minced garlic, grated ginger, and crushed red pepper flakes until the sugar is dissolved.
Pour the sauce mixture evenly over the chicken and vegetables in the slow cooker.
Cover and cook on low for 4 hours, or until the chicken is cooked through and tender.
Remove the chicken breasts and vegetables from the slow cooker and place them on a serving platter.
In a small bowl, mix the cornstarch and water to create a slurry.
Pour the liquid remaining in the slow cooker into a saucepan and bring to a simmer over medium heat.
Whisk in the cornstarch slurry and cook for 2-3 minutes until the sauce thickens.
Pour the thickened sauce over the chicken and vegetables.
Garnish with sliced green onions and serve immediately, ideally over steamed white rice or your choice of side.