At this point, you should be able to obtain Onion Knight job via a side quest. There is certain condition in order unlock this quest. 

Now first go to speak to any MogNet as Luneth (whoever at the top of the menu screen will appear as an avatar), you should have 4 mails from Topapa as follow:

• How are you?
• Ur
• The Children of Ur
• The Children are in Trouble!

If you don't see "The Children are in Trouble!", make sure you have collected 25% of bestiary.

• Yeah you heard Topapa, some kids are missing! Now your objective is to help find them. But before that, go talk to Topapa first in Ur, he will tell you that the kids have gone to Alter Cave.
• After giving Topapa a visit, go to Altar Cave, walk a few steps, and watch a show then kill 3 Bombs.
• After saving the kids, they will give you a piece of Crystal which happens to be Onion Knight's job.