Preparation Time: 1 1/2 hour
In a large pot, bring the stock or broth and cream to a gentle simmer over medium heat. With a sharp chef's knife, trim the ends off the celery root and remove the peel, carving with the shape of the bulb to retain the curve. Cut each bulb lengthwise into quarters. Add to the pot, cover and cook, turning occasionally, until the pieces are just tender, 20 to 30 minutes.
Cut the potato into ¼” thick slices.
Heat the oven to 350 degrees. Remove the pot from the heat. Using a slotted spoon or tongs, transfer the celery roots to a cutting board (reserving the liquid in the pot) and let the pieces sit until cool enough to handle. Cut them into 1/4” thick slices.
Layer the potato and celery root slices in a 10-inch gratin or other baking dish, overlapping them slightly. Bring the liquid in the pot to a boil so that it thickens a bit, then whisk in the mustards, salt and pepper to taste. Pour the mixture evenly over the potato and celery root, covering it completely. Sprinkle the cheese evenly over. Bake 35 to 40 minutes, until the liquid is bubbly and the cheese is melted. Serve hot.