The Tuscans will tell you they invented onion soup—one of the best-known versions of this soup, carabaccia, was jotted down in a 1500s cookbook. Serve it with or without the eggs, but I think they add great sustenance to this otherwise very simple soup. If you want richer flavor, you can use chicken stock, or fry some pancetta or sausage pieces and add them to the soup. In the springtime, a version of this soup includes fresh peas (frozen will do) and fava (broad) beans.
You can make the soup ahead with everything except the eggs and Parmesan and simply add these once you have reheated the soup.
This is edited slightly from my cookbook "Florentine: The True Cuisine of Florence," published by Hardie Grant Books, 2016 (in the original I call for making your own vegetable stock, but for ease, I didn't specify—but obviously, for such a simple recipe, use a good quality one, whichever way you decide to go).
In a large, heavy-bottomed stock pot, gently saute the onion in the olive oil with a good pinch of salt and pepper and sage leaves on lowest heat. Let them sweat very gently without letting them colour to release their sweetness, about 30 minutes. If they begin to get dry or stick just add a splash of the vegetable stock.
Add the stock to cover and turn heat up until it begins to simmer. Keep heat on medium and let the soup cook 30 minutes. Check for seasoning and add salt or pepper as needed.
About 5 minutes before soup is ready, sprinkle half of the cheese over the top of the soup, crack the eggs into the pot and cover with the rest of the cheese and cover, without stirring. The whites should be cooked and the yolks runny—about 5 minutes.
Serve the soup with an egg in each bowl along with the bread. If desired, sprinkle over extra Parmesan, a grating of black pepper and a drizzle of olive oil.
The Australian-Japanese cookbook author has lived in Florence (where a visit to a cheese farm once inspired her to start a food blog) for over 10 years with her Tuscan sommelier husband and two kids. Her third cookbook, Tortellini at Midnight, is out now.