Wash the spinach and blanch in boiling salted water for about 2 minutes. Drain, rinse and drain very well. Finely chop the leaves.
Coarsely grate the pecorino. Whisk with the ricotta and eggs until well combined. Season with salt, nutmeg, pepper and oregano and stir in the spinach.
Preheat the oven to 180°C, gas 4. Grease a 24 cm tart tin with butter.
Brush each filo pastry sheet with a little oil and arrange in the tin slightly offset, leaving the edges of the pastry hanging over the rim.
Spoon in the spinach filling and spread level. Fold the overhanging pastry towards the centre to cover the filling.
Brush with the remaining oil and sprinkle with sesame seeds.
Bake for 40 minutes until the pastry is golden and crisp. Leave to cool for about 5 minutes and serve cut into slices.