A “Ficelle Picarde” is a crêpe with a ham and mushroom filling.
A speciality of the Picardy region of France, it is a crêpe rolled up into a thin cylinder.
Inside, it has a thin slice of ham and a duxelles of mushroom, along with a spoonful of a cream sauce. The sauce will be perhaps Mornay, but never Béchamel.
The rolled-up crêpes are sprinkled with some grated cheese such as gruyère, and then baked.