Combine the flour and salt in a large bowl. Make a well and crack in the egg and pour in the milk and 1 tablespoon of oil.
Whisk until you have a smooth batter. Set aside.
Heat the butter in a frying pan and add the onions. Fry until soft.
Add the mushrooms and balsamic vinegar and cook until it has reduced.
Add the spinach and wilt and set aside.
Wipe a frying pan with oiled kitchen paper and heat over a moderate heat. Ladle the batter into the centre of the pan and tilt the pan so the batter spreads thinly around the pan. Cook undisturbed for 30 seconds.
Flip the pancake and heat the other side for 30 seconds. Once cooked, put the pancake on a plate and cover with foil to keep it warm. Repeat with all the remaining batter.
Assemble the pancakes by adding a line of the onion mixture and then the grated cheese. Fold and serve.
Make sure you pour only a very thin layer of batter in to the pan. These are meant to be thin, light and delicate crepe type pancakes. Too much batter would make them stodgy.
You can freeze these pancakes for up to 3 months. Make sure to put a layer of baking paper between each pancake though, to stop them from sticking.
Make sure your pan is heated, but not so hot that it is smoking otherwise the pancakes will burn. It's best to brush the pan with oil, so it gets a light coating of oil.