In a bowl, add the lean beef, breadcrumbs, olives, egg, oregano and salt. Use your hands to mix everything together. Roll into walnut-sized meatballs and set aside.
In a large pot, warm the oil over medium heat. Add the onion, celery, carrot and salt and cook for 5 minutes to soften the vegetables. Stir in the garlic, courgette and oregano and cook for another 30 seconds.
Add the vegetable stock and drop each meatball into the soup. Bring the pot to a simmer and cook, covered, for 15 minutes or until the meatballs are cooked through.
Stir in the olives, parsley and lemon juice, then serve.