Make Sauce: Whisk together pineapple juice and cornstarch in a bowl. Add ketchup, sugar, soy sauce, oil, and garlic and whisk to combine. Add pineapple and season with salt and pepper.
Make Meatballs: Combine chicken, scallions, egg, breadcrumbs, and garlic in a second bowl. Season with salt and pepper. Form mixture into 10 meatballs.
