Preheat the oven to 350 degrees F (175 degrees C).
Mix together ground beef, bread crumbs, onion, milk, egg, and salt in a large bowl until well combined. Roll mixture into balls and arrange onto a baking sheet.
Bake in the preheated oven until meatballs are browned on the bottom, about 25 minutes. Transfer to a slow cooker.
Stir together brown sugar and cornstarch in a medium saucepan and place over low heat. Add reserved pineapple juice and vinegar; stir until smooth. Bring to a boil for 1 full minute before removing from heat; pour over meatballs in the slow cooker.
Add pineapple chunks and green bell pepper to the slow cooker.
Cook on Low until flavors are thoroughly combined, 3 to 4 hours.
