2: Cut the potatoes and celery into slices. A mandolin is ideal for this, but you can also do it with a knife.
3: Pour 3 tablespoons of water into a large saucepan, followed by 1 litre whole milk (the water will prevent the milk from sticking to the bottom). Add celery and potato slices, 1 bayleaf, salt and pepper.
4: Cook over low heat until the celery is tender. Note: As always, when cooking in milk, keep an eye on the boiling point, as overflowing can happen very quickly.
5: Remove the vegetables from the pan with a skimmer and place in a food mill. Grind everything...
6: ...to obtain a dry purée, which you return to the saucepan emptied of milk and placed over medium heat.
7: Whisk in 100 g butter, cut into small pieces, then 50 g cream. Turn off the heat.
8: Finish by adding 2 tablespoons mustard (preferably "à l'ancienne" for the aesthetic appeal of the grains), check the seasoning and your celery purée is ready.
Remarks
You can use the cooking milk, with its slight celery flavour, later for another recipe, such as a potato gratin or béchamel sauce.