This is the tool for you if you want to save time when preparing vegetables.
People with professional knife skills may not need one of these. However, if you are not one of them, and would still like some speed and consistency in the dicing of onions or slicing of potatoes, tomatoes etc. then you need a mandoline grater.
There are many different mandolins on the market and watch out because price and usefulness of mandolins does not always go together. This one may look simple, but it does the job well.
Try this one if you want something a bit more advanced: