To make the meatballs, preheat the oven to 400 degrees F. Line a large baking sheet with parchment paper or aluminum foil.
In a large bowl, combine the beef, egg, panko, onion powder, garlic powder, ½ teaspoon salt, and ¼ teaspoon black pepper.
Mix well and shape the mixture into 12 meatballs.
Transfer the meatballs to the prepared pan and bake for 20 to 25 minutes, until browned and cooked through (160 degrees F on a meat thermometer).
While the meatballs cook, make the sauce.
In a large skillet, combine the brown sugar, vinegar, ketchup, soy sauce, and 3/4 cup of the pineapple juice (once drained, you should have about 1 cup of pineapple juice from the can).
Set the pan over medium-high heat and bring to a simmer.
In a small bowl, whisk together the cornstarch and remaining ¼ cup pineapple juice. Whisk until the cornstarch is completely dissolved.
Add the cornstarch mixture to the pan and return to a simmer.
Reduce the heat to medium low, add the bell peppers, and simmer for 8 to 10 minutes (or until the meatballs finish cooking). The mixture will have thickened and reduced slightly.
When the meatballs have finished cooking, add them to the sauce with the pineapple chunks.
Simmer for 3 minutes to heat through. Top with green onions and serve.