Preheat the oven to 400°F (200°C).

Line a baking sheet with parchment paper and lightly grease it with cooking spray or oil.

In a large mixing bowl, combine ground beef, breadcrumbs, finely chopped onion, crushed pineapple, teriyaki sauce, garlic powder, salt, and black pepper until just mixed.

Shape the mixture into meatballs about 1–1.5 inches in diameter using a melon baller or cookie scoop for uniform size.

Place the meatballs on the prepared baking sheet, leaving space between each one for even cooking.

Bake in the preheated oven for about 25 minutes, or until the meatballs reach an internal temperature of 160°F (70°C).

While the meatballs bake, heat a small saucepan over medium heat and add 1/4 cup extra teriyaki sauce, simmer for 5–7 minutes until slightly thickened.

Once the meatballs are cooked, remove them from the oven, drizzle with the thickened teriyaki sauce, and garnish with chopped green onions.