For the mushroom quiche, prepare a shortcrust pastry from the ingredients:
Put flour on a baking board. Make a well in the center. Add egg and a pinch of salt. Spread cold butter in pieces on top.
Quickly knead all the ingredients together to form a dough. Wrap in plastic wrap and leave in the refrigerator for half an hour.
In the meantime, brush, clean and slice the mushrooms.
Peel and finely chop the onion.
Sauté the onion in olive oil and add the mushrooms. Continue to sauté until all the liquid that has escaped has evaporated.
Mix the crème fraîche with the eggs, add the mushrooms and season with salt and pepper.
Grease a cake tin (26 cm ø). Roll out the short pastry on a lightly floured work surface and line the pan with it, pulling up the edges. Prick the pastry base a few times with a fork.
Spread the filling on the dough base and bake the Schwammerlquiche in the preheated oven for about half an hour.