Onions are plants belonging to the Genus Allium. Several species may be referred to as onions, but Allium cepa is the most widely known and cultivated. Long, spear-like leaves bunch together into a fleshy base, which is often partially or fully underground. The base is referred to as a 'bulb'. Sometimes it is confused with a root, but the true roots are short and sprout from the underside of the bulb. The bulb is used as nutrient storage for the plant during Winter.
The bulb is the most commonly consumed portion, but the green leaves can also be eaten, especially when young and tender. While humans can readily eat onions in a variety of ways, they are toxic to other animals such as dogs, cats, and others.
Onion soup appears on the table at the Hysteric Tribe's Freya'sday Eve feast. It is the traditional dish for the occasion. In Book 4, Hiccup falls into a giant cauldron of onion soup when trying to sneak into the Hysteric Great Hall on a quest.
It is also mentioned that Toothless really likes onion soup.
Later, when Hiccup escapes from the Hysterics, he "pelted them all with onions" and other food from the table.