In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, garlic powder, onion powder, salt, and pepper. Mix until well combined.
Shape the mixture into small meatballs, around 1 inch in diameter, and set aside.
Preheat the oven to 400°F (200°C).
Place the meatballs on a baking sheet lined with parchment paper. Bake for 18-20 minutes or until they are cooked through and browned on the outside.
Meanwhile, in a large skillet over medium heat, add a splash of oil (if desired) and sauté the diced onion and bell peppers until they're soft, about 5 minutes.
In a mixing bowl, whisk together the soy sauce, ketchup, apple cider vinegar, brown sugar, and sesame oil (if using).
Once the meatballs are done, add them to the skillet with the sautéed vegetables. Pour the sauce mixture over the meatballs and bring to a gentle simmer.
If you prefer a thicker sauce, mix the cornstarch with a little water to create a slurry and stir it into the sauce, allowing it to cook for another 2-3 minutes until it thickens.
Stir in the pineapple chunks and let everything cook together for an additional 2-3 minutes until heated through.
Garnish with sliced green onions before serving.
Notes
Serve over rice or quinoa, and garnish with sesame seeds.