Make meatballs: Tear bread into small pieces and place in a bowl. Pour over 1/4 cup cold water. Set aside for 5 minutes to soften. Combine mince, ham, egg yolk and parmesan in a bowl. Squeeze water from bread and add bread to mince mixture. Mix well. Season with pepper. Shape tablespoons of mixture into balls and place on a plate. Cover and refrigerate until ready to use.
Heat oil in a large saucepan over medium heat. Add onion, carrot and celery. Cook, stirring occasionally, for 5 minutes or until vegetables soften. Add zucchini and cook for 2 minutes.
Add stock and bring to the boil. Reduce heat to low. Add pasta and simmer, uncovered, for 10 to 15 minutes or until pasta is tender.
Carefully drop meatballs, 1 at a time, into soup and stir gently. Simmer for 5 minutes or until meatballs are cooked through. Season with salt and pepper. Ladle soup into warmed bowls. Top with parmesan. Serve.