I got this Job when i defeated Salamander(Boss) in Molten Cave. Then speak to the Moogle. Read the 4th Mail about Ur (Missing Kids). Then talk to Topapa in Ur then go to the Cave where you started in the beginning of the game. Defeat the 3 Bombs then you get the Onion Knight Job.
The Bombs can self-destruct which can hurt alot. it did 300-400 damage to my Knight. and might able to do little more damage to others so becareful. Just use the highest Ice spell , Ice Staves or Ice Battle item on the Bombs and on Salamander(Boss). and always have a White Mage to heal your party.
Talk to the Moogle everytime you do something new.
If you get the Onion Knight Job don't level up as a Onion Knight until you are at LeveL 92 becuase they have sucky stats. but if you're LeveL 92 then level up as a Onion Knight then all your Stats will start to rise very high. Making them the best Job in the game.
They can use any equipment but not the Class only eqiupments.and can cast LV1-8 White and Black Magic Spells.