Prep the sauce: In a bowl or measuring cup, whisk together pineapple (with its juice), ketchup, soy sauce, brown sugar, and vinegar.
Add garlic and ginger. Stir until smooth.
Layer the chicken: Place the chicken thighs in a slow cooker, Instant Pot, or 9x13-inch baking dish. If using veggies, scatter bell pepper and onion around the chicken.
Dump and go: Pour the sauce evenly over the chicken.
No need to stir.
Cook it: Slow cooker: Cook on Low for 4–5 hours or High for 2–3 hours, until chicken is tender and easily shreds.
Instant Pot: Pressure cook on High for 8 minutes (thighs) or 10 minutes (breasts), then natural release for 5 minutes and quick release the rest.
Oven: Cover with foil and bake at 375°F (190°C) for 30–35 minutes, or until the chicken reaches 165°F (74°C).
Thicken (optional): Remove chicken to a plate. Stir cornstarch with water to make a slurry. Add to the hot sauce and simmer (in a saucepan or sauté function) for 2–3 minutes until glossy and slightly thickened.
Finish: Shred or slice the chicken and return it to the sauce.
Taste and adjust with a splash more soy for salt or a dash of vinegar for brightness.
Serve: Spoon over rice or noodles. Top with green onions and sesame seeds if you like.