In a large bowl, use hands to mix the pineapple, ground beef, breadcrumbs, onion, egg, salt, and pepper. Shape mixture into 40-45 meatballs using about 15 mL (1 tbsp.) for each one. Place on a parchment-lined baking sheet.
Bake in oven for about 15 minutes or until internal temperature reaches 70°C (160°F).
In a small bowl, dissolve cornstarch in water.
Combine the reserved pineapple juice, white wine vinegar, brown sugar, and ketchup in a large saucepan over medium heat. Add the cornstarch mixture and heat gently until boiling or until mixture thickens.
Add meatballs and coat with sauce. Sprinkle meatballs with green onion, if using.