Steam cook the sliced potatoes.
Arrange in a gratin dish.
Prepare a béchamel sauce, by melting the butter and adding the flour.
Mix well then pour the milk in one go and bring to the boil.
Then add the cream, the sugar, mustard and the salt.
Top the potatoes with this sauce and leave to cook in the oven at a medium heat for about 10 minutes.
Serve with the croustisoy, pan-fried for about 3 minutes on both sides.