Peel both the celery root and the parsnip, using a knife to remove the tougher and more gangly parts of the celery root. Use a mandoline to shave them both into very thin slices. You can use a knife to do this, but the mandolin will result in slices with much more even thickness.

Grease the bottom of a gratin dish, pie plate, or other small-medium baking dish. Place down one even layer of celery root, then sprinkle with a generous amount of salt and pepper, then top with a layer of parsnip. Sprinkle the parsnip layer with more salt and pepper, along with a tiny bit of nutmeg and a sprinkle of gruyere. Repeat this process until all the vegetables, cheese and nutmeg are gone. Be sure to finish with a layer of cheese. Pour cream over and press down to release any air bubbles.

Preheat the oven to 350 degrees. Cover the dish with foil and bake in the oven for about 45 minutes.

While the gratin is baking, prepare the topping. Heat the olive oil and butter in a sauté pan over medium heat and add in the minced garlic and then the breadcrumbs. Toss and move the breadcrumbs around the pan until they are evenly browned, about 3-5 minutes.

When the gratin is almost cooked through, remove the foil, top with the breadcrumbs, and put back in the oven for an additional 15 minutes. Remove the gratin from the oven and allow it to rest for about 10 minutes before serving so that it can set up.