Place chicken chunks in the bottom of a 6-quart slow cooker.
Whisk together pineapple juice, brown sugar, soy sauce, and minced garlic in a medium bowl until sugar dissolves.
Pour sauce mixture over chicken, coating all pieces evenly.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until chicken reaches 165°F and is fork-tender.
Add pineapple chunks and red bell pepper (if using) during the last 30 minutes of cooking.
For thicker sauce, mix cornstarch with 2 tablespoons cold water and stir into slow cooker. Cook uncovered for 15-20 minutes until thickened.
Serve over rice and garnish with sliced green onions and toasted sesame seeds.