Preheat an oven to 400°F. Butter a 13-by-9-inch baking dish.
Layer 1/2 of potatoes in the baking dish, season with 1/3 of the salt and pepper to taste, and top with 1/3 of the grated cheese.
Next, create another layer using all the sliced celery root. Season with 1/3 of the salt and pepper to taste and top with another 1/3 of the cheese.
Do a final layer, using the remaining potatoes. Season with remaining salt, pepper to taste, and remaining cheese.
Pour the cream or milk over the gratin, cover tightly with foil, and bake for one hour.
After one hour, check gratin piercing vegetables with a fork. If they are tender, remove foil, and bake another 15 minutes until golden. If the vegetables are not yet tender, cover with foil and cook another 15 to 20 minutes or until tender. Depending on the age and moisture of your vegetables, you have to repeat this cycle a few times. Once tender, remove foil and bake for another 15 minutes until golden.
Let stand for 10 minutes, garnish with optional chives or parsley, and enjoy!