Onion soup is a type of vegetable soup with sliced onions as the main ingredient. It is prepared in different variations in many different countries, the most famous of which is the French onion soup or Parisian onion soup. Because of the affordable ingredients, it has primarily been a dish for the poor for a long time.
Common for all variations of onion soup is the use of thinly sliced or chopped onions soaked in fat, and a liquid base such as water or broth, possibly including white wine, after which the soup is cooked for a while so that the onions lose their strong flavour and the soup gains a sweet, spicy flavour. In many recipes the soup is thickened with flour or egg yolks.