Clean the spinach thoroughly and Devein by removing the thick tough veins from the large leaves.
Bunch the leaves together on a chopping board and roughly chop into thin strips.
You may need to run your knife through a couple of times to get everything chopped up.
Set aside.
Heat a cast iron skillet (or any other oven-proof skillet) on medium-low heat.
Add the oil & butter, then the onion.
Cook the onion, stirring, for 3 minutes until starting to soften & yellow.
Add the garlic and cook a further minute or 2 until fragrant.
Add the chopped spinach to the skillet and immediately cover it.
Wait 2 minutes, then remove the lid and stir to evenly distribute the onion and garlic mixture through the spinach.
As soon as the spinach is wilted, lightly season the mixture with salt & pepper.
Remove from the heat and set aside to cool slightly while you prepare the rest of the filling.
In a bowl, combine the ricotta and feta cheese.
Use a large fork or a potato masher to mash the cheeses together till well mixed.
Add the eggs and mix through.
Add the chopped mint, then the spinach and onion mixture (just make sure it isn’t too hot or it will scramble the eggs!
Better to leave it until it stops steaming completely & is almost room temperature).
Mix everything together well, then pour it all back into the same skillet you used to cook the spinach.
Set aside.