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

First, it requires you to complete at least 25% of bestiary. You should have done this in the previous section.

Second, go talk to any MogNet as Luneth (there is one MogNet in Gysahl) or whoever at the top of the menu screen will appear as an avatar, then read all mails from Topapa as follow, they appear one at a time:

• 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. If not, roam around the world map or caves to collect bestiary until you reach 25%.

• Now your objective is to help find the missing kids. But before doing that, go back to Ur and talk to Topapa, he will tell you that the kids have gone to Altar Cave.
• After giving Topapa a visit, enter Altar Cave, then proceed to B2. Walk to the end and a scene will trigger. There, you'll have to kill 3 Bombs.
• After saving the kids, they will give you a piece of Crystal which happens to be Onion Knight.