In the original Final Fantasy III, Onion Knights were the default class. When the game was remade for the Nintendo DS, Freelancers became the default class and Onion Knights were unlocked via a secret quest involving the Mognet.
When this remake was ported to iOS devices, Mognet was removed. However, the Wikipedia article for Final Fantasy III mentions this:
A port of the remake for iOS was released on March 24, 2011. In which the game-play and graphics where improved sound was also remastered But the Mail/Mognet to other players was removed. And the "Onion Knight" was found in another quest.
What is that quest, and what are the steps one needs to take to unlock the Onion Knight?