French onion soup is a soup of onions, gently fried and then cooked in meat stock or water, usually served gratinéed with croutons or a larger piece of bread covered with cheese floating on top. Onion soups were known in France since medieval times, but the version now familiar dates from the mid-19th century.

An onion, also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus Allium. The shallot is a botanical variety of the onion which was classified as a separate species until 2011. Its close relatives include garlic, scallion, leek, and chive.