01 - Whisk together pineapple juice, soy sauce, ketchup, brown sugar, rice vinegar, ginger, garlic, sesame oil, and black pepper in a medium bowl until fully combined and sugar dissolves.
02 - Place chicken thighs in the slow cooker, arranging pieces in an even layer. Pour the prepared sauce over the chicken, turning pieces to coat thoroughly.
03 - Cover and cook on low heat for 4–6 hours until chicken reaches internal temperature of 165°F and is tender enough to shred easily with a fork.
04 - Transfer cooked chicken to a serving platter. Whisk cornstarch with water in a small bowl until smooth, then stir into the hot sauce in the slow cooker. Cover and cook on high for 10–15 minutes until sauce thickens. Return chicken to pot and coat evenly.
05 - Place chicken over steamed rice on serving plates. Spoon thickened sauce generously over chicken. Sprinkle with green onions, sesame seeds, and fresh pineapple slices if desired.