Prep the chicken: Pat the chicken dry and season lightly with salt and pepper. This helps it brown a touch in the cooker and stay flavorful.
Layer the base: Add the sliced onion and bell peppers to the bottom of the slow cooker.
Scatter the chicken pieces on top.
Make the sauce: In a bowl, whisk together soy sauce, ketchup, honey (or brown sugar), rice vinegar, sesame oil, garlic, ginger, red pepper flakes, and 1/2 cup reserved pineapple juice.
Pour and cook: Pour the sauce over the chicken and vegetables. Cover and cook on Low for 4–5 hours or High for 2–3 hours, until the chicken is tender.
Add pineapple: Stir in the pineapple chunks during the last 30 minutes of cooking to keep them bright and juicy.
Thicken the sauce: Stir the cornstarch slurry (2 tablespoons cornstarch + 2 tablespoons cold water) into the slow cooker. Cover and cook on High for 10–15 minutes, until the sauce is glossy and slightly thick.
Taste and finish: Adjust seasoning with a splash more soy sauce for salt, vinegar for brightness, or honey for sweetness.
Garnish with sesame seeds and green onions.
Serve: Spoon over warm rice or cauliflower rice. Add extra sauce over the top for the full sweet-and-savory effect.