In case there are others like me that didn't realize:

Feeding a Thavarian Onion to your chocobo during training at the sables does NOT instantly rank up your chocobo from 10 to 11. It increases the level CAP to 11. After the feeding you need to go level your chocobo to the new cap.

If you feed your chocobo another Thavarian Onion before hitting the new cap, your onion will be wasted and the cap will NOT increase.

Given that this means you're looking at a LOT of time between onion feedings for new cap increases I feel like only getting 1 onion per seed planting is kind of unfair, especially for large FC groups. With a small house it takes nearly 2 months to get one full bed of Thavarion Onions.