Something worth remembering is that vegetable matter cooks at a higher temperature than the proteins. So, it is questionable how much banana leaf flavour will be imparted during the sous vide time.
Raw onion has very strong and volatile flavour compounds. Onion gives some of these compounds off at room temperature. The onion itself would not start cooking until the temperature gets to about 80C. The strong raw onion flavour all came from the low temperature volatiles.