Clean the mushrooms and cut them into slices. 

Sauté in olive oil together with the garlic and add the spring onions, finely chopped, so that they get tender. Remove the garlic. 

Heat the margarine, add the flour while stirring and add the hot milk in order to make a light béchamel sauce. Remove from heat, add the egg, the feta cheese and the herbs and season with salt and pepper. Finally add the mushrooms with the onions and mix. 

Brush a 22 cm tart pan with the margarine and line it with the shortcrust pastry filo sheet. Spread the mixture in it. 

Bake the tart in a preheated oven at 200°C for 40-45 minutes.  

Remove from the oven, let it cool for 10 minutes and then serve.