Preheat the oven to 180°C / 350°F / Gas mark 4.
Tip the flour and a pinch of salt into a large bowl.
Make a well in the centre and crack in the eggs.
Whisk the flour and the egg, while gradually adding the milk until there is a smooth, lump-free batter.
Allow to rest at room temperature.
In a large frying pan, heat the butter, olive oil and add ½ of a teaspoon of salt.
Fry the mushrooms until they turn soft and golden.
Remove from the heat, stir in half the cheese, most of the parsley, the mustard and the crème fraîche.
Mix until combined, then season with black pepper.
In a separate frying pan over medium heat, add a little sunflower oil then use a pastry brush or piece of kitchen roll to wipe the surface of the pan so that it’s lightly oiled evenly.
Pour enough batter into the pan so that it’s coated with a thin layer of batter.
Cook on a medium-low heat until the pancake looks set and is golden on the underside.
Use a spatula to move the pancake to one side then flip over.
Alternatively slide the pancake (golden side down) onto a plate, then place back into the pan.
Cook for about 45 seconds more, or until the other side is golden.
Layer the cooked crepes on a plate while finishing the rest, brushing the pan with oil between each one.
Divide the mushrooms filling between the crêpes (reserving a tablespoon for over the top of each one) and scatter over some of the remaining cheese, reserving a little for the top.
Roll up each one and pop onto a baking tray.
Add the remaining cheese on top and transfer to the oven for 5 minutes to warm through.
Remove, and scatter with the rest of the parsley before serving.