Preheat the oven to 375°F (190°C).

In a large bowl, combine ground beef, breadcrumbs, onion, egg, and milk.

Season the mixture with salt and pepper.

Shape the mixture into small meatballs, about 1 inch in diameter.

Place the meatballs on a baking sheet lined with parchment paper.

Bake in the preheated oven for 20 minutes, or until browned.

While meatballs are baking, prepare the sauce by combining pineapple juice, vinegar, ketchup, and brown sugar in a saucepan.

Bring the mixture to a simmer over medium heat, stirring occasionally.

Once the meatballs are done, transfer them to the saucepan and coat with sauce.

Serve warm and enjoy!