In a large mixing bowl, combine the lean ground beef, plain breadcrumbs, milk, large egg, finely grated onion, minced garlic, grated ginger (if using), salt, and black pepper. Mix gently.

Using your hands, scoop the meat mixture and form into 1.5-inch meatballs, aiming for 24-28 total. Place on a baking sheet.

Heat a tablespoon of vegetable oil in a large skillet over medium-high heat. Sear the meatballs for 3-4 minutes until browned on all sides.

Preheat your oven to 375°F (190°C).

In a saucepan, combine pineapple juice, light soy sauce, brown sugar, rice vinegar, and ginger over medium heat. Stir gently.

Prepare cornstarch slurry in a small bowl, then stir it into the simmering sauce until thickened.

Spread rice evenly in a large baking dish. Arrange seared meatballs and pineapple chunks on top.

Drizzle thickened sauce over meatballs. Bake in preheated oven for 20-25 minutes until meatballs are cooked through.

Garnish with chopped green onions before serving.