Begin by finely slicing both your onions and potatoes.
Bring a large pan of salted water to the boil and add in sliced potatoes. Boil until just soft, around 7-8 minutes then drain and set to one side.
In a frying pan add your butter and gently brown over a medium heat until it reaches a golden caramel colour. Add in your onions and continue to cook over a medium to low heat so the onions gently caramelise, around 10-12 minutes.
Once caramelised remove from the heat and add in the sour cream, grated Grana Padano and chopped oregano plus season to taste. Stir and set to one side.
Preheat your oven to 190 degrees Celsius.
Take either one pie dish around 22cm or two medium sized smaller options and line with puff pastry. Prick the base with a fork and press the edges of the pastry into the case. Trim the top of the pastry off.
Next fill each pastry case with the potato and sour cream mix and finish with an abundant scattering of Grana Padano for extra flavour and crunch. Bake for around 25-30 minutes or until golden.
Serve warm with a fresh crispy salad or as part of a larger meal.