Preheat the oven to 180˚C, gas mark 4. Butter a 30cm x 20cm gratin dish. Melt the butter in a large pan, add the onion and garlic then cook over a medium heat until softened.
Add the cream and mustard, season with the salt and plenty of black pepper, then simmer for 5 minutes. Take the pan off the heat.
Cut the celeriac and potatoes into thin slices. Mix them with the cream mixture in the pan, then pour into the gratin dish and flatten the top. Bake for 1 hour 15 minutes-1 hour 30 minutes until completely tender to the point of a knife.