Peel the potatoes, cook or steam until tender with a pinch of salt, drain.
Cut off the bottom of the celery, pluck off the leaves, keep only the stalks, wash well and peel the top with a vegetable peeler until the strings are gone.
Cut them into equal pieces the same length as a head of endive.
Steam them for 5 minutes until tender, pat dry.
Divide them over the 8 slices of ham, roll up and place in a baking dish.
Are you doing it without the ham? Then cut a washed leek completely in half lengthwise, steam until tender and roll these strips around the celery.
Do not place them too close together so that there is still plenty of potato sauce in between.
Mix the potatoes with the milk, butter, pepper, salt, and a pinch of nutmeg with an immersion blender, add half of the cheese, you should get a fairly runny puree.
Pour over the ham rolls, sprinkle the rest of the cheese on top and gratinate for 25 minutes at 210°C.
Tips
Be sure to also try rolls with turnips, celeriac or leek instead of celery.