Some people have reported being able to skip this step depending on the amount of time elapsed when starting the next quest.
This timer seems to be related on when you come into posession of the Old Ring during Mission 3-1.
If you have a Carbuncle's Ruby in one of your many storage areas AND have not yet started the Summoner job quest, you may get the opening cutscene for I Can Hear a Rainbow. Check the quest log to see if "Know One's Onions" is marked as completed.
There are reports that the Cat Burglar has been loitering around the Ghosty House in Windurst Walls. This looks like a job for the Star Onion Brigade! Gather four wild onions so Kohlo-Lakolo and his gang can create the perfect trap!
SPOILER WARNING: Click here for additional details about this story line.