In a large bowl, mix together ground beef, breadcrumbs, egg, chopped onion, parsley, garlic powder, salt, and pepper until well combined.
Form 1 1/4-inch balls with the meat mixture.
Heat enough oil to coat a large skillet. Cook meatballs over medium heat, turning to brown evenly, until thoroughly cooked, about 10 minutes.
In a separate bowl, whisk together the remaining ingredients, except for the green onions. Stir until there are no lumps.
Pour the sauce over the cooked meatballs in the skillet. Lower the heat and let the sauce simmer and thicken. Garnish with green onions, if desired, and serve hot.