Step 1

Put the chard into a large saucepan, pour over a kettle of boiling water, then simmer for 4 minutes until bright and just tender. Drain in a colander and leave to cool.

Step 2

Return the saucepan to a medium heat, melt 50g butter with 1 tbsp oil, then fry the onion with a pinch of salt for 5 minutes. Add the remaining oil, the leeks and celery, then sweat down for 10 minutes. Set side.

Step 3

Preheat the oven to 190ºC, gas mark 5. In a large mixing bowl, whisk together the ricotta, cheese, garlic, oregano, lemon zest and juice, then crack in the eggs and whisk to a smooth mix. Using a clean tea towel, squeeze the chard over the sink to remove excess water, then add to the egg mix with the onion, leeks and celery. Stir well for an evenly mixed filling, then season.

Step 4

Melt the remaining butter in a separate small pan. Brush a deep 24cm-square baking tin (or approx 22x26cm roasting tin) with some melted butter. Brush 1 pastry sheet with butter and lay across the tin horizontally, ensuring it hangs evenly over the sides. Place another buttered sheet of filo on top, this time vertically, then place a third buttered filo sheet into the tin diagonally.

Step 5

Pour the filling on the filo pastry base, then layer 3 more sheets of filo on top, brushing with butter each time. Crinkle the filo hanging over the edges into the middle, ruffle the last filo sheet into the centre, brush with butter all over, then sprinkle with nigella seeds.

Step 6

Bake for 25-30 minutes until golden brown on top. Remove from the oven and leave to cool for 10 minutes before serving. Delicious hot or cold.