For the pancakes, sift the flour with a pinch of salt into a mixing bowl, then crack in the egg and mix well. Add the milk and 150ml/5fl oz water, then beat well until smooth and well combined. Set aside while you make the filling.
For the filling, heat the oil in a frying pan and fry the onion until softened, then add the mushrooms to cook through. Season well with salt and pepper, add the baby spinach and cook for 2 minutes until it wilts, stirring frequently. Set aside while you cook the pancakes.
To cook the pancakes, heat a little olive oil in a large frying pan, pour in a ladleful of batter and swirl around until it thinly covers the base of the pan. Fry for about a minute until the underside is set and beginning to turn golden-brown. Turn over using a spatula and cook the other side.
While the pancake is still in the pan, spread half the crème fraîche over one side (if using), scatter over half of the mushroom and spinach mixture, and sprinkle with parsley and chives (if using). Fold the sides of the pancake over the filling and lift onto a serving plate.
Repeat with the remaining batter and filling to make another pancake (if you have any batter leftover, fry-up another pancake for dessert). Serve with salad leaves.