Lightly oil a roasting tin (ours was about 25 x 35cm and 4cm deep) and line with baking parchment. Mix the eggs, cheeses, soured cream or crème fraîche, spinach and some seasoning together in a bowl.

In another bowl, mix together the oil, yogurt and baking powder. Heat the oven to 200C/180C fan/gas 6.

Cut the filo sheets in half so they are roughly the size of the tin. Put one filo sheet in the tin and evenly brush over 1-2 tbsp of the yogurt mixture using pastry brush. Top with another filo sheet, brush over more yogurt mixture and repeat until you’ve used half the filo.

Pour over the cheese filling, then gently spread it to the edges using a spatula. Continue layering the rest of the filo sheets and yogurt mix. Spread any remaining yogurt mix over the top of the final filo sheet.

Cut the pie into squares (either nine or 12, depending on how big you want the pieces to be). Will keep frozen for up to a month. Defrost completely in the fridge overnight before cooking. Top each square with a small knob of butter, then bake for 20 mins. Reduce the oven to 180C/160C fan/gas 4 and bake for another 25-30 mins until browned on top. Leave to cool slightly before serving warm. Will keep covered and chilled for up to two days.