French onion soup is a type of soup usually based on meat stock and onions, and often served gratinéed with croutons and cheese on top. Although ancient in origin, the dish underwent a resurgence of popularity in the 1960s in the United States due to a greater interest in French cuisine. French onion soup is usually served as a starter. Onion soups have been popular at least as far back as Roman times. Throughout history, they were seen as food for poor people, as onions were plentiful and easy to grow.

Source: The Soup Spoon