Pour the stock into a saucepan and bring to the boil. Reduce the heat and simmer for 2–3 minutes. Beat the eggs, Parmesan, breadcrumbs and nutmeg in a bowl and season generously. Gradually whisk 2 ladlefuls of hot stock into the eggs.
Reduce the heat under the saucepan then slowly stir the egg mixture into the stock until smooth, making sure that the temperature remains moderate, as the egg will curdle if the soup boils. Gently simmer for 2–3 minutes until piping hot.
Tear the basil leaves into pieces and add to the pan, then ladle the soup into bowls. Serve with extra Parmesan to grate over the soup to taste.