1. Preheat the oven to 350 °F
2. Peel the celery root and potatoes and slice them thinly. Put them in a bowl and mix with oil, salt, and pepper. Toss well with your hands so that the oil coats the slices evenly.
3. Mix Greek yogurt with vegetable broth, mustard, a pinch of nutmeg, salt, pepper, and half of the cheese.
4. Place the celery root and potato slices alternately in an ovenproof dish. Pour the yogurt mixture over the vegetables, making sure everything is well coated. Gently shake the dish to level the mixture and rearrange the slices if they move.
5. Cover with aluminum foil and bake for 20 minutes.
6. Mix breadcrumbs, the remaining cheese, and a pinch of thyme.
7. Remove the dish from the oven, remove the foil, and sprinkle the breadcrumb mixture. Return to the oven for another 40 minutes. The surface should be golden brown. Let it rest for 10 minutes before serving.