Heat the oven to 180C/. Heat two tablespoons of oil in a deep skillet and, on medium heat, sauté the onion for eight minutes without browning.

Add the chard stems and the celery, cook for four minutes, stirring occasionally, then add the chard leaves, raise the heat and cook, stirring, for four minutes, until wilted.

Add the spring onion, rocket and herbs, cook for two minutes and transfer to a colander.

Once cool, squeeze out as much liquid as you can, transfer to a bowl and mix in the cheeses, zest, eggs, salt, pepper and sugar.

Lay out a sheet of filo and brush it with olive oil. Cover with another sheet and repeat until you have five layers of filo brushed with oil and covering an area large enough to line the sides and base of a 22cm pie dish, with extra overhanging the edges. Line the dish with filo, fill with the herb mix and fold the excess pastry over the edges of the filling, trimming to create a 2cm border.

Make another five-layer filo piece as before and place on top. Scrunch up the pastry a little to create a wavy, uneven top, and trim the edges so it just covers the filling.

Brush with more oil and bake for 40 minutes, until the filo turns golden brown.

Serve warm or at room temperature.