Place 1/2 of the chicken thighs in a large ziplock plastic freezer bag and sprinkle with 2 tablespoons cornstarch. Seal the bag and shake until chicken is evenly coated.
Heat 1 tablespoon of oil in a large nonstick skillet over MEDIUM-HIGH. Add coated chicken and cook 3 minutes on each side or until lightly browned. Transfer browned chicken to a 5 to 6 quart slow cooker.
Repeat coating and browning process with remaining chicken, cornstarch, and oil.
Drain and reserve the juice from the can of pineapple chunks. Cover and refrigerate drained pineapple chunks for later.
Combine finely chopped onion, rice vinegar, 1/4 cup of the reserved pineapple juice, ketchup, honey, soy sauce, ginger, garlic, and salt in a medium bowl. Pour sauce over chicken in slow cooker. Cover and cook on HIGH for 3 hours or on LOW for 6 hours, until chicken is tender and cooked through. Stir the chopped bell pepper into the chicken and sauce and spoon reserved pineapple chunks over the top. Cover and cook on HIGH for an additional 30 minutes, or until bell pepper is fork tender.
Serve over rice garnished with green onions.