Roll out the tarte flambée dough with the baking paper on a baking sheet and brush with the crème fraîche. Leave about 1 cm free all around.
Halve or quarter the leek depending on the thickness of the stick, cut across into very thin strips and spread on the dough.
Cut the mushroom heads into milli-thick slices and place them next to each other on the leek.
Season with salt and pepper and sprinkle very thinly with the grated cheese.
Put the tray on the lowest rail in the oven and bake for about 15-20 minutes until the desired brown color. Every oven bakes differently, so adjust the temperature and baking time to the experience with your own device.
In terms of quantity, it is enough for about 8 people as a snack, at least 2 of which should be full.