Combine the mince, breadcrumbs, egg and a third of the onion and half the oregano in a bowl. Season with salt and pepper. Using clean hands, shape tablespoons of the mixture into 20 meatballs.
Heat half the oil in a large, deep frying pan over medium-high heat. Cook meatballs, turning occasionally, for 5 minutes or until browned. Transfer to a plate.
Heat the remaining oil in pan over medium heat. Add the garlic and remaining onion. Cook, stirring, for 3 minutes or until onion is softened. Add the wine. Cook, stirring, for 1 minute. Add the tomato, stock and remaining oregano. Bring to the boil. Reduce heat. Simmer, covered, for 10 minutes.
Return meatballs to pan with olives. Simmer, stirring occasionally, for 5 minutes or until meatballs are cooked through. Serve saganaki sprinkled with feta and parsley.