RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Onion Mixture
The Old School RuneScape Wiki also has an article on Onion Mixture.

Onion Mixture is an unfinished Cooking ingredient which is made by using a onion with a bowl, while having a knife in your inventory. Creating an onion mixture grants no Cooking experience.

A tomato can be added to the mixture in order to produce Onion and Tomato Mixture.

The Onion Mixture is one of only two uses for Onions other than when they are required for quests. The only other use for onions in RuneScape Classic is in having Aggie make you yellow dye from them.