Butter for greasing the baking dish
1 pound celeriac, peeled, halved, sliced about 1/8 inch thick
1 pound baking potatoes, peeled, sliced about 1/8 inch thick
Salt and freshly ground black pepper
1 cup grated Gruyère or domestic Swiss cheese, divided
1/2 teaspoon dried thyme
2 cups chicken, beef, or vegetable stock
2 tablespoons butter
Preheat the oven to 350° F. Grease a 2-quart baking dish with butter.
Place the celeriac and potatoes in alternating layers in the baking dish, seasoning every few layers with salt and pepper.
At about the halfway point, add 1/3 cup cheese in an even layer; sprinkle with the thyme. Continue with the celeriac and potatoes, until you have used all of your slices (don’t go all the way to the top edge; leave a little room to allow the liquid to boil).
Pour the stock over the celeriac and potatoes. Dot with butter. Cover with foil and bake for 45 minutes. Remove the foil and bake for 15 minutes more. Sprinkle the remaining 2/3 cup cheese over the top layer, add several grindings of fresh pepper, and bake until the cheese turns golden, about 15 minutes. Let stand for 10 minutes before serving.