01 - Arrange the chicken thighs or breasts in an even layer at the bottom of the slow cooker insert.
02 - In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, ketchup, rice vinegar, garlic, and ginger until the sugar dissolves completely. Pour the mixture evenly over the chicken.
03 - Distribute the sliced onion, bell pepper pieces, and pineapple chunks over the chicken and sauce.
04 - Cover and cook on LOW setting for 4-5 hours, or until the chicken reaches an internal temperature of 165°F and is tender enough to shred easily.
05 - Transfer the cooked chicken and vegetables to a serving platter using a slotted spoon. Cover loosely with foil to maintain warmth.
06 - In a small bowl, whisk the cornstarch and water until smooth and no lumps remain.
07 - Pour the cooking liquid from the slow cooker into a medium saucepan. Bring to a simmer over medium heat, then gradually whisk in the cornstarch slurry. Continue whisking and cook for 2-3 minutes until the sauce thickens enough to coat the back of a spoon.
08 - Pour the thickened sauce over the chicken and vegetables. Garnish with sliced green onions and sesame seeds if desired. Serve immediately over steamed rice.