Preheat the oven to 350°F.

Peel and slice the potatoes, sweet potatoes, and celeriac into very thin slices (about 0.04–0.08 inches). Try to keep the slices even so they cook uniformly.

Place the sliced potatoes and celeriac in a large saucepan with the cream and a pinch of salt. Heat over low heat and stir regularly until the cream just begins to simmer.

Transfer the potatoes and celeriac with the cream to a deep baking dish. Season generously with pepper.

Cover the dish with parchment paper and place it in the middle of the preheated oven. Bake for 40–45 minutes, checking after 40 minutes that the potatoes are cooked through and tender.

Serve warm. Enjoy!