In a large heavy base frying pan heat oil and add mushrooms. Cook mushrooms, stirring continuously, until they start to soften and release liquid. Add garlic and continue to stir mushrooms until all liquid has been released and evaporated and you have a dry pan. This will take around 20 minutes over a medium/low heat.
Remove mushrooms from the stovetop and leave to the side to cool completely.
While mushrooms are cooling add feta, ricotta, parmesan, eggs spring onions, parsley and oregano to a large bowl, mix. Once mushrooms have cooled add them to your cheese mixture and mix until completely combined. Season with some ground salt and pepper.
Brush a 22cm round spring form tin with melted butter. Line with baking paper, a circle in on the base and strips around the side. Brush over the baking paper with more melted butter once tin is lined.
On a chopping board place one sheet of filo pastry. Brush the filo pastry with melted butter. As this will eventually form the lid of your pie there is no need to brush the entire filo sheet. Just paint a filled circle on your filo, which is slightly larger than your spring form tin.
Place another layer of filo on top of your butter brushed sheet, and brush with butter as you did before. Continue doing this until you have used half the packet of filo pastry.
Once you completed the above step place your spring form tin on top of your layers of filo. Cut a circle about 1cm wider than your tin out of the filo. Keep the circle covered for later use, discard the leftover cut filo.
Now start to layer your spring form tin with filo pastry. Making sure to brush each layer well when you place it in your tin. As your filo sheets will most probably be a rectangle shape, you will need to lay them alternately in your tin (one length wise and then the other widthwise) to ensure maximum coverage. Do not brush the last layer of filo pastry with butter.
Place your mushroom mixture into your filo lined tin. Press down into the edges and level the top with the back of a spoon.
Place the filo lid you have set aside on top of your pie and press it down until it is flat.
With a knife or kitchen scissors cut the filo pastry that is over hanging your tin. Making sure to leave enough that you can fold it over the top to seal the lid of your pie. Don’t be precious (as you can see I haven’t) about getting it all even. Once you fold it over nobody will be able to tell. Plus having it a little rustic looking adds to the charm of this pie.
Place in oven and bake for 45 minutes. The top will puff up slightly and your pie should be a lovely golden brown.