01 - Whisk together pineapple juice, soy sauce, ketchup, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes in a medium bowl until sugar dissolves completely.
02 - Arrange chicken thighs in the bottom of the slow cooker. Distribute chopped bell peppers, sliced onion, and drained pineapple chunks evenly over the chicken.
03 - Pour the prepared sauce mixture evenly over all ingredients. Cover and cook on LOW setting for 5-6 hours or HIGH setting for 3-4 hours until chicken reaches internal temperature of 165°F and is fork-tender.
04 - Combine cornstarch and cold water in a small bowl, whisking until smooth and lump-free.
05 - Stir the cornstarch slurry into the slow cooker during the last 30 minutes of cooking. Leave lid partially off to allow sauce to thicken properly, stirring occasionally.
06 - Transfer chicken and vegetables to a serving platter, spooning thickened sauce over top. Sprinkle with sliced green onions and toasted sesame seeds. Serve immediately over steamed rice or coconut rice.