1 large celeriac peeled and thinly sliced
1 Tbsp fresh lemon juice
salt and freshly ground pepper
2 Tbsp butter, plus additional for buttering
4 oz gruyère cheese, grated (1 cup)
2 large potatoes, unpeeled, thinly sliced
1 cup heavy cream
1/2 cup plain dried bread crumbs
Boil potatoes in salted water, covered, until tender. Drain.
Preheat the oven to 375 F. Generously butter a medium size gratin or baking dish.
In a large saucepan, toss the celeriac with the lemon juice, add salted water to just cover, Simmer over low heat until just tender, 5 to 6 minutes. Drain.
In the prepared dish, make a layer of half of the celeriac. Dot with 1/2 Tbsp of the butter. Sprinkle with 1/4 cup of the cheese and season with salt and pepper. Top with half of the potatoes, dot with 1/2 Tbsp more butter, sprinkle with 1/4 cup cheese, and season with salt and pepper. Repeat the layers. Pour in the cream and spread the bread crumbs on top.
Bake in the preheated oven until bubbling hot and golden brown, about 30 minutes. Serve very hot.