01 - Whisk together pineapple juice, soy sauce, ketchup, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes in a medium bowl until well combined.
02 - Arrange chicken thighs in the bottom of the slow cooker. Layer bell peppers, onion, and pineapple chunks evenly over the chicken.
03 - Pour the prepared sauce evenly over the chicken and vegetables, ensuring all pieces are coated.
04 - Cover and cook on LOW for 5-6 hours or HIGH for 3-4 hours until chicken is tender and reaches an internal temperature of 165°F.
05 - Mix cornstarch with cold water in a small bowl, stirring until completely dissolved and smooth.
06 - During the final 30 minutes of cooking, stir the cornstarch slurry into the slow cooker. Mix gently to incorporate evenly.
07 - Leave the lid off for the last 30 minutes to allow the sauce to thicken properly.
08 - Transfer to serving plates and garnish with sliced green onions and toasted sesame seeds. Serve hot alongside steamed rice.