You will start by mixing all the above mentioned ingredients for the Crêpes to a smooth liquidy dough. Leave it standing in a cool place for 30 minutes.
In the meanwhile chop the Onion and Mushrooms fine and fry the onions on slow heat first alone till translucent, throw in the Mushrooms right after that and cook it all till the water from the mushrooms dissolved and the mixture well cooked is. Season it and keep aside.
Now you'll have to cook the béchamel sauce by adding the butter to a pot to melt.
Mixing in the flour, stirring continuously.
Quickly, pour in little by little the milk and continue whisking the whole mix on the heat till it all thickens to a smooth sauce.
Keep it aside on a warming plate because we will need it warm later on.
Take out your crêpes dough and heat up a frying pan with little Oil. Add to the hot pan 1 big Ladle of dough and quickly turn the pan around so that the dough evenly spreads. Cook the crêpes each side 2-5 minutes till it slightly reaches a slight golden color, take it out and keep aside. Do that with the rest of the dough and cook 4 more crêpes batches.
To assemble the Crêpes Ficelle picarde, place the crêpes open, add a ham slice on top and top it all with the mushroom onion mix. Now roll the crêpes gently in and do that with all the other crêpes.
The last step, to finnish the recipe, involves covering the crêpes with the hot Bechamel sauce first and garnishing with the grated cheese and chives at the end.