Preheat oven to 400°F. Line a baking sheet with parchment paper and brush with oil. Steam the celery root slices for about 7 minutes. They should be soft but not mushy.
In a soup plate, combine water and starch. Stir until the starch is dissolved. In a second soup plate, mix all the breading ingredients.
Dip the celery slices in the water mixture and coat with breading. Place on the baking sheet and spray with oil. Bake for 20 minutes, carefully flipping the schnitzel after 10 minutes.