In a large bowl, combine ground beef, breadcrumbs, milk, egg, chopped onion, salt, and pepper. Mix gently until just combined; do not overmix.
Shape the mixture into 16 small meatballs, about 1 inch in diameter.
Heat vegetable oil in a large skillet over medium-high heat. Brown meatballs on all sides, about 2-3 minutes per side. They do not need to be fully cooked at this stage. Transfer to a plate and set aside.
In the same skillet, add ketchup, apple cider vinegar, brown sugar, soy sauce, and pineapple juice. Stir to combine and bring to a simmer.
Return the meatballs to the skillet, spooning sauce over them. Cover and simmer for 15-20 minutes, or until meatballs are cooked through.
Stir in the cornstarch slurry and cook for an additional 2-3 minutes, until the sauce has thickened.
Serve hot over cooked rice, garnished with diced pineapple if desired.
Notes
For a thicker sauce, increase the cornstarch slurry slightly. Leftovers can be stored in an airtight container in the refrigerator for up to 3 days.