First make the crepe mixture - this is easiest with a hand blender but can be done by hand. Whisk the egg and salt together. Add the flour and ¼ cup milk and mix until smooth. Add up to ¼ cup more milk, to give a fairly runny batter. Leave to rest while making the filling.
Remove any long stems from the mushrooms, cup them in half and slice. Cut leek in half lengthwise then cut in medium-thin slices.
Melt the butter over a medium heat in a small-medium pan. Add the leeks and cook for approx 5min to allow them to soften, stirring regularly.
Next, add the mushrooms and thyme and allow mushrooms to cook, another 5 or so minutes, then add the turkey/chicken.
Add the flour and stir in so it absorbs all the butter. Meanwhile, heat your frying pan/skillet for the crepes.
Reduce the heat a little then add half of the milk, stir so it is mixed in then add the remaining milk and stir in. Add the cheese, stir so it melts and mixes through then turn off the heat. Set aside while you cook the crepes (or warm ready-made ones)
Pour approx ¼ cup of the crepe mixture into the warm frying pan/skillet. Do this with the pan off the heat and at the same time swirl it round the pan so it covers all of the base. Fill any gaps with a little extra batter.
After a couple minutes, loosen the edge and once the edge starts to curl up and go crisp, turn the crepe over with a spatula/palette knife. Cook a couple more minutes on the other side.
Remove to a plate, put a large spoonful of turkey/chicken mixture to one side of the crepe and roll it up. Repeat with the remaining crepe and turkey mixtures - you should get around 4 depending on size.