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.