Preheat your oven to 180 degrees celsius fan-forced.
Place the parmesan and tasty cheese (cut into 4cm cubes) into your Thermomix bowl and mix for 8 seconds, speed 7 to grate. Transfer to a separate bowl and set aside until needed.
In a clean Thermomix bowl, add the onion (cut in half), garlic and olive oil and mix for 4 seconds, speed 6 to grate. Cook for 3 minutes, 90 degrees, speed 2.
Add the feta, eggs, sour cream, grated cheeses, nutmeg, a pinch of salt and pepper to your Thermomix bowl and mix for 20 seconds, speed 4.
Add the baby spinach leaves and mix for 10 seconds, speed 3 on REVERSE to combine.
Place a piece of the filo pastry onto your bench and brush it with a little melted butter. Top it with another sheet of filo pastry and again brush this with the melted butter. Repeat until you have used all of the sheets of pastry.
Carefully transfer the pastry into a 20cm square baking dish and cut away any excess pastry which is hanging over the sides.
Pour the filling into the dish and gently smooth the surface. Place the spinach and Cheese Pie into the oven and bake for 40 minutes or until golden.