Heat the oven to 180°C. Peel the potatoes and the celeriac, rinse, then slice thinly, say 2–3mm thick, and put them into separate bowls. Wash both bowls of sliced vegetables until the water is clear. Drain the vegetables and roughly dry.
Cover the bottom of a large baking dish with the sliced celeriac, lightly overlapping the slices. Strew the cooked chard over the celeriac. Cover the chard with the sliced potato, again overlapping the slices slightly. Season the cream with salt, black pepper and nutmeg and pour this over the layers of sliced vegetables. Dot with butter. Cook in the oven for 45 minutes, until browned and bubbling at the edges. Let the gratin sit for 10 minutes or so before eating.