Preheat Oven: Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or lightly grease it.

Prepare the Meatballs: In a large bowl, combine the ground beef, breadcrumbs, milk, egg, chopped onion, minced garlic, soy sauce, salt, and black pepper. Mix until all ingredients are well combined.

Form the Meatballs: Using your hands, form the mixture into small meatballs, about 1 inch in diameter. Place the meatballs on the prepared baking sheet.

Bake the Meatballs: Bake in the preheated oven for 20-25 minutes, or until the meatballs are cooked through and browned on the outside.

Make the Sauce: While the meatballs are baking, combine the brown sugar, apple cider vinegar, ketchup, soy sauce, garlic powder, ground ginger, and the reserved pineapple juice in a medium saucepan. Bring the mixture to a simmer over medium heat, stirring occasionally.

Thicken the Sauce: In a small bowl, whisk together the water and cornstarch until smooth. Gradually add the cornstarch mixture to the saucepan, stirring constantly, until the sauce thickens. Add the pineapple chunks to the sauce and stir to combine. Remove from heat.

Combine Meatballs and Sauce: Once the meatballs are done baking, transfer them to the saucepan with the sweet and sour pineapple sauce. Gently stir to coat the meatballs evenly with the sauce.

Serve: Serve the sweet and sour pineapple meatballs hot, as an appetizer or over rice for a main dish.