Preheat the oven to 190°C / 374°F.
Lay open the stack of filo on your working surface. Cover it with a tea towel so it won't air-dry.
In a small bowl mix together the butter and olive oil. Keep it next to you to use for brushing the pan as well as the filo.
Take a round 31 cm (12-inch pan) and grease it well with the olive oil and butter mix.
Lay 6 filo sheets inside the pan in order to cover its bottom and all of its edges all around. Brush each filo you add to the pan with butter-olive oil mix.
Add half of the filling on top and spread it evenly with a spoon.
Take 2 more filo sheets and place them on top of the filling again greasing each one of them.
Add the remaining half of the filling and again spread it and smoothen it with a spoon.
Now add the remaining sheets of filo on top. Ruffle them a bit with your hands in order to fit inside the pan and not hang out of it. Drizzle each one with the oil-butter-mix and tap with the pastry brush to spread it gently.
Overlap the bottom layers of filo (the ones that hang out of the pan) in order to create a ruffled pie edging. Grease the edging the same way you did the top of the pie.
Scar and divide the pie into 8 pieces. Try not to cut the bottom layers of filo.
Bake for about 15 minutes until the pie gets a nice golden brown color on top.
Then drop the temperature to 170°C / 338°F. and bake for another 35 to 40 minutes. Shake the pan to see if the pie moves inside it that means it's baked at the bottom as well.
Let the pie sit at room temperature for at least 30 minutes before serving.