For the crust, combine oat flour, ground almonds, corn starch and salt.
Add olive oil and cold water and mix with your hands until dough forms.
Wrap in plastic film and chill for 30 minutes.
Preheat the oven to 180ºC (370ªF)
If you choose to make two individual tarts, divide the dough into two balls.
Press the dough evenly onto the bottom tin or over a floured surface, roll it with a rolling pin and cover the base of the tin.
Trim the extra dough, prick with a fork to prevent rising and bake it 10 minutes.
Remove from the oven and set aside.
To make the filling, beat eggs, oregano, nutmeg, salt and pepper and if using feta cheese, crumble it and mix all together.
Heat olive oil in a pan and sautee mushrooms until golden. Add spinach and leave it wilt for a couple of minutes.
Pour over the egg mixture and then over the prebaked base.
Put into the oven and bake for 30 minutes until the filling is set and golden.
Leave it cool before serving.
Serve it with a tomato or avocado salad