You dont need a stable to use the Thavnairian Onion to incress lvl cap. The stables just let you feed your choco every hour for exp and allow you to change the color of your choco. You'll need a Thavnairian Onion for every lvl past 10 so a total of 10 Thavnairian Onion's. Give your choco an Onion after you reach each lvl cap.