Onions is a sack that contains between one and ten onions. They are used namely in the Farming skill. If a player has an empty sack in their inventory and clicks on it while onions are also in the inventory up to ten onions will be placed in the sack. To remove onions from the sack requires right clicking the sack of onions and selecting the menu option that removes one onion at a time. Sacks can be bought from many farming stores, such as the one in the chicken farm north of Port Sarim. A maximum of ten onions can be put into a sack. Once ten onions are in the sack and a player tries to put more, it will be full. This works the same way with cabbages and potatoes, except for cabbages in Draynor Manor.
Note that cabbages will go into an empty sack before onions if a player has cabbages in their inventory. As such it is recommended to 'use' the sack on one onion at which point the player can click on the partially full sack of onions to continue filling it.
Onions are useful for the Temple Trekking, as players must feed five NPCs with any piece of food to stop them from turning into ghasts in one of the events. One sack of onions can complete two events, and they are cheaper than cabbages and potatoes. On the other hand, potatoes have the advantage of not possibly being accidentally eaten by the player. Note that a Ghast can turn an entire sack of onions into a piece of rotten food.