Onion is a kind of crop.
In the smurf medias, onion is one of the food that smurfs eat. In the comic book "Salad Smurfs", Handy Smurf has turned in a living onion due to Chef Smurf putting too much potion on the crops. In the cartoon show episode, Crying Smurfs, Papa Smurf's potion was accidentally poured into an onion by Weepy Smurf, and to make matters worse - it's smell cause the Smurfs to cry uncontrollably like Weepy himself.
Onions are a plant in the Smurfs' Village game, that with giving the third most experience points, 175, only losing to Pumpkin and Sarsaparilla, are really only worth 6 coins, although they really pay out 22 coins. That is because it costs 16 to grow them, so the other 12 hour plant, Watermelon is worth more, worth 24 coins, and Watermelon almost give out that much experience, so it is more recomended to plant Watermelon, despite being avalible at level 15, unless you are trying to get experience faster and do not care about your coins.