In sauté pan, heat the oil over medium heat below smoking. Add the other lemongrass stalk loosely chopped to the oil and cook it in the oil until the oil is fragrant with the scent of the lemongrass, remove from the heat and strain out the lemon grass and set the oil aside to cool.
Once the celery root and onion are tender and soft, while still warm add them to a vita mix. The lemongrass that was in the steamer should be soft but not soft enough to puree so using a gloved hand ring out as much moisture from the lemongrass as possible into the celery root and onion mixture.
Puree until smooth, while the vita-mix is running stream the lemongrass scented oil in to the puree emulsifying it into the puree, season with salt.