Prepare the oven and filo: preheat the oven to 180 °C (fan if possible) and line a large baking tray with baking paper. Take the filo sheets out of their packaging and cover them with a slightly damp tea towel to prevent them from drying out while you work.
Cut the filo strips: stack the filo sheets on the work surface and cut them into 3 equal strips lengthwise. You will get 3 piles of strips. Immediately place two of the piles back under the damp towel so they stay supple.
Prepare a double strip: take one strip of filo, lightly brush it with olive oil using a pastry brush, then place a second strip on top to reinforce it.
Shape the triangles: place a heaped tablespoon of filling at one end of the strip, near the bottom edge. Fold one corner of the pastry over the filling to form a triangle, then keep folding upwards, alternating left and right like a little flag, all the way to the end of the strip (see the folding diagram above). Place the triangle on the baking tray, seam side down. Repeat with the remaining strips and filling.
Brush and sprinkle: brush the tops of the triangles with a little olive oil, then, if you like, sprinkle with sesame or nigella seeds.
Bake: bake for 18 to 22 minutes, until the triangles are nicely golden and crisp; keep an eye on them towards the end of the baking time, as filo browns quickly.
Serve: let the Spanakotyropitakia rest for a few minutes before serving so you don’t burn yourself, then enjoy them warm as a starter, as part of a mezze, or with a fresh salad.
(With 10 to 12 sheets of filo, you will usually get around 15 to 18 triangles, depending on the width of the strips and how much filling you use for each one.)