Firstly, combine the flour and salt in a large bowl. Make a well and crack in the egg and pour in the milk and 1 tbsp of oil. Whisk until you have a smooth batter.
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 batter.
Heat the butter in a frying pan and add the onions. Fry until soft. Add the balsamic vinegar and cook until it has reduced.
Move the onions to one side of the pan and add the spinach. Heat until the wilted down.
Finally, assemble the pancakes by adding a line of spinach, topped with the onions and then the grated cheese. Garnish with parsley.