Onions are a vegetable mentioned briefly in the How to Train Your Dragon Franchise.
Onions (Allium cepa) are plants with thick, but hollow, blade-like leaves. They are most known, however, for their bulb, or a specialized underground stem. This bulb is a round mass of tightly packed fleshy modified leaves which the plant uses to store nutrients. Onion bulbs are often white, but varieties exist with outer layers of different colors such as purple-red and yellow.
Onion bulbs are a nutritious vegetable. They are also known for irritating the eye. As a defense mechanism of the plant, when a fresh onion is damaged (or cut up for cooking), it releases a gas that stimulates the nerves in the eye and causes a burning or stinging sensation. This further stimulates the eye to produce tears.
Onions are used primarily as food in real life, and in the Franchise, though mention of them is very minute.
In the episode, "Viking for Hire", Gobber is left with little to do in his smithy, since Berk is not fighting dragons anymore, and weapons do not need to be forged at the pace they once were. He tries several new activities with little success. At one point Hookfang acts erratically and endangers the village. Stoick calls out to Gobber that he is needed, but, feeling defeated, Gobber thinks he is not needed anymore and cries a little, blaming it on chopping onions. Ultimately, Gobber saves the day by yanking out a diseased tooth from Hookfang that was causing him problems.
Onions are mentioned briefly in the episode "The Longest Day". Fishlegs is paranoid due to lack of sleep and mutters that "They're gonna peel my skull open like an onion" to get at his dragon knowledge.
Gobber briefly mentions onions when he suggests making a stew with a dead mouse.