In a large bowl combine the meatball ingredients (beef, onion, egg, oats, milk, salt and pepper) then shape into meatballs, about 1 1/2 inches in size. Place them in a single layer on a baking sheet.
Bake the meatballs (without the sauce) for 30-35 minutes. Or until centres are not pink.
The Sweet and Sour Sauce:
While the meatballs are baking make the sweet and sour sauce.
In a saucepan combine all the sauce ingredients (sugar, cornstarch, water, vinegar, soy sauce and ketchup) and bring to a low boil.
Whisk and cook until the mixture thickens.
Pour the sauce over the cooked meatballs, and put in a 300 degree F oven for about 20 minutes before serving.