Prepare the crust. Mix flour with salt. Incorporate butter to the flour, using your hands or using a pastry processor, until crumbs are formed. Add the egg and incorporate until the dough is homogeneous. Knead the dough and form a ball from it. Wrap it with plastic and refrigerate for at least 30 minutes.
In the meanwhile, prepare the spinach filling. In a large flat-bottomed frying pan heat oil over medium-high heat. Cook onion,for about 5 minutes until soft. Add the mushrooms and spinach and cook until any liquid has evaporated. Add salt and pepper and remove from the heat and allow to cool.
Preheat the oven to 220C. Roll out the dough and fit the crust to a 10 inch tart pan. Punch it with a fork and bake it for 15 minutes. Remove and let it cool. Turn the oven to 190C.
In a bowl beat eggs with sour cream, salt pepper and cheese. Layer the spinach over the crust and pour the egg mixture on top.
Bake for 35-40 minutes, and serve warm or cold. The rest of the quiche can keep up t0 3 days if refrigerated.