Peel the celeriac and cut into very thin slices on the cutting machine. Mix the remaining ingredients with a hand blender and season high with salt and pepper. Place the slices celeriac in the mixture for 15 minutes so they can absorb some moisture. Lubricate an appropriate roast ing sled with a thin layer of oil and place the celeriac slices evenly into it. Press very well and make sure it's just under the cream. Cover with baking paper, lightly press and bake for 25 minutes at 180°C. Remove the baking paper and weight and bake it for another 10 minutes at 180°C. Put it away under light pressure for 4 hours, so you can cut tight slices from it. Heat in the oven just before serving.