Cook the spinach: Transfer the olive oil and the leeks to a large pot and cook over medium high heat, stirring occasionally, until the leeks are glossy and translucent. Add the spinach and stir until it wilts down and reduced in volume. Reduce heat to a gentle simmer and cook until all the liquids are evaporated (if you're in a hurry: after cooking the spinach for 10 minutes, drain all the liquids from the pot and continue with the recipe).
Make the filling: Remove the pot from the heat and let it cool down for a few minutes. Add the crumbled feta cheese, freshly grated black pepper and a pinch of salt. Mix to combine. Taste and adjust the salt and the pepper.
Layer the phyllo: Grease your pan with oil and cover the half with one phyllo sheet and the other half with a second phyllo sheet, making sure the sheets overlap each other (if the sheets are a little ruffled they will trap air between them and they will bake better and become crispier). Brush with oil very gently without pressing the phyllo, and repeat until you have used about 2/3 of the phyllo.
Fill and cover: Fill with the filling and cover with the rest of the phyllo, brushing with oil each sheet. Tuck the phyllo between the pie and the edges of the pan or fold the rim inwards and crimp to secure.
Score and bake: Sprinkle the surface with some water and score into 12 pieces. Bake in the middle/low rack of your oven for 60 minutes or until golden brown on top . Let it cool down for 30 minutes and serve warm or cold.
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