Peel off the celery root, cut in quarters and slice into 1/4 inch slices.
Put them in a pot, add milk and butter, and cook slowly for about 20 minutes (be careful as milk has a tendency to spill if it is boiling too rapidly).
Preheat oven to 425 F. Butter an oven-proof dish and arrange gently the slices of the cooked celery root at the bottom of it.
In a bowl mix together cream, egg yolk, 2/3 cup of cheese, and milk from cooking the celery root.
Pour the mixture over the celery root slices and cover on top with the remaining cheese.
Bake for about 20-30 minutes, until the cheese melts and forms a gold crust.
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