Place the chicken breasts in the bottom of the slow cooker. Add the sliced red bell pepper and yellow onion on top.
In a bowl, whisk together the reserved pineapple juice, ketchup, rice vinegar, brown sugar, soy sauce, and minced garlic until the sugar dissolves.
Pour the sauce over the chicken and vegetables. Add the drained pineapple chunks and stir gently to combine.
Cover and cook on low for 5 to 6 hours until the chicken reaches an internal temperature of 165°F.
Remove the chicken, shred or slice it, then return it to the slow cooker.
Stir the cornstarch with 2 tablespoons of water to make a slurry. Add it to the slow cooker and stir. Cook on high for 15 more minutes until the sauce thickens.
Serve the chicken and sauce over cooked rice. Top with sliced green onions.