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.
02 - Arrange chicken thighs in the slow cooker bottom. Layer bell peppers, onion, and pineapple chunks evenly over the chicken.
03 - Pour the prepared sauce uniformly over all ingredients in the slow cooker.
04 - Cover and cook on LOW setting for 5-6 hours or HIGH for 3-4 hours until chicken reaches 165°F and is fork-tender.
05 - Mix cornstarch with cold water in a small bowl, stirring until completely smooth and lump-free.
06 - During the final 30 minutes, stir cornstarch mixture into the slow cooker. Replace lid and continue cooking.
07 - Remove lid for the last 30 minutes to allow sauce to reduce and thicken properly.
08 - Transfer to serving plates, garnish with green onions and sesame seeds. Serve immediately with steamed rice.