Peel and slice the onions. In a large saucepan, soften them in olive oil until they are lightly browned. Add the white wine, then 1.5 litres of water. Cook for 45 minutes.
Finely chop the parsley and add to the soup.
Off the heat, bind the soup with the egg yolks (be careful though – the temperature mustn’t exceed 80°C).