In a heavy based pan add 50g of butter and cook the sliced onions gently at a low heat for 40 mins. Stir regularly until soft and caramelised. Once cooked set to the side to cool.
Preheat the oven to 200°C.
Lay a 50cm strip of greaseproof paper out on a clean work surface, scrunch it up then flatten out again. Brush the paper lightly with melted butter. Lay 1 sheet of filo pastry onto the buttered grease proof paper and brush gently with melted butter.
Repeat this process until you have 5 layers. Rotate each sheet by 25 degrees to create a star pattern. It does not matter if you overlap the paper as the edges will be folded in.
Whisk eggs and add cheddar, feta cheese, caramelised onions and dry oregano. Mix together and gently fold through the potato seasoning with salt and pepper to taste.
Slide the greaseproof and filo into a 30cm frying pan so that the edges spill over. Push down into the sides of the pan.
Pour the filling mix into the pan and gently scrunch over the edges of filo towards the centre to start to almost enclose it like a pie. This does not need to be perfect and flat as the ruffled pastry will go really nice and crispy in the oven.
Brush the surface with melted butter and sprinkle with some flakey sea salt and a little more oregano over the top. You can also trim away any excess paper.
Place the pan over a high heat for 2 minutes to cook the bottom of the pastry.
Transfer to the oven and bake for 25-30 mins or until golden brown and the filling has set.
Remove from the oven and allow to cool for 10 mins before slicing.