Celery root gratin recipe: not exotic but still unknown

Ingredients

• 1 large celery root about 2 libras,
• 1 1/3 cup milk,
• 1 egg yolk,
• 1/2 cup creme fresh, or heavy whipping cream,
• 1 cup Gruyère, Fontina, or Cheddar cheese, grated,
• 3 tablepsons butter,
• salt and white pepper.