Prep the slow cooker: Lightly grease the crock or use a liner.
Place the chicken in a single layer if possible.
Mix the sauce: In a bowl, stir together pineapple chunks with juice, soy sauce, ketchup, brown sugar or honey, garlic, ginger, and rice vinegar. Add a pinch of red pepper flakes if you like heat.
Combine: Pour the sauce over the chicken. If using onion, scatter it on top.
Cook: Cover and cook on Low for 4 to 5 hours or High for 2.5 to 3 hours, until the chicken is tender and reaches 165°F.
Shred or slice: Remove the chicken to a cutting board.
Shred with two forks or slice into chunks, then return it to the crock.
Add bell pepper: Stir in the sliced bell pepper. Cook on High for another 20 to 30 minutes to soften slightly while staying crisp-tender.
Thicken the sauce (optional): Stir the cornstarch slurry into the crock. Cook on High for 10 to 15 minutes until the sauce lightly thickens.
Taste and adjust: Add a splash more soy for salt, a teaspoon of sugar for sweetness, or a little vinegar for brightness, as needed.
Serve: Spoon over warm rice.
Garnish with sliced green onions and sesame seeds if you like.