Into a small saucepan add your ingredients for the mushroom filling. Cook on medium heat for around 10 minutes or until your mushrooms no longer hold any moisture and your onions are soft and brown. Set this mixture aside to cool while you prepare your filo.
Remove your filo from the package and lay flat on a flat surface. If you're unfamiliar with filo are work a bit slower with it then set a damp cloth on top of the filo sheets to avoid drying.
Grab your butter and place a pastry brush into it.
Grab your first piece of thin filo and place onto a large, wooden, cutting board. Brush with butter and repeat around 6 times. You may need more butter depending on how liberal you are. Cut your filo sheet into quarters. Place your mixture into the middle of each piece and gather all four corners at the top and pinch to stick together. Note: Filo is very delicate so move slowly if you feel nervous, you want to avoid the pastry from tearing.
Place in the freezer for 10 minutes while your oven preheats to 400F.
Cook for 15 minutes (depending on your oven) or until golden brown and perfectly crispy.
I suggest doubling or trippling this recipe.Do not over stuff, you want to make sure they stay closed while baking.Will last for a couple days in the fridge but best enjoyed fresh.They will last up to 3-4 weeks in a tight sealed container in the freezer. Cook from frozen in the oven.