Take Thief up to at least level 15 (45 if you're feeling bored and eager to farm a lot) and go after Goblin Thugs in East Sarutabaruta and Outer Horutoto Ruins. The tower you want to farm in and around is at (J-11). These sell (on Bismarck) for 30k a stack and they're very quick to farm. The goblins are level 1~8, so you should be able to level up on them while also farming the onions.

Or if you're a Bastokan kind of guy you can farm Beehive Chips from bees around Gustaberg. These sell (on Bismarck) for 13k a stack.

You could also try crafting, but it's not really necessary. You need high level crafts to really start profiting from it. They are initial gil sinks.

When you reach level 99 Dynamis solo is great gil.