Preheat the oven to 180°C/Gas 4.
Wring out the spinach by squeezing it in your hands. Put in a large bowl and set aside.
Heat the olive oil in a frying pan and gently soften the onion for 8-10 minutes, add to the spinach.
Crumble the feta into the spinach. Add the 3 eggs and oregano. Mix together.
Lightly oil a 9” spring form pan. Lay out the pile of filo and cover with a damp tea towel.
Working quickly, line the pan with one sheet of filo (let the excess drape over the side of the tin) then brush it lightly with some vegetable oil. Repeat with the other 4 filo sheets.
Spoon the spinach mixture into the filo lined tin and fold over the excess and brush the top with oil.
Place the pan in the oven and bake for 30-40 minutes, rotating halfway through.
Remove from the oven and rest for 10 minutes before removing the ring of the pan.
Cut into slices and serve immediately.
I served it with a Greek salad.