1.Preheat oven to 180°C. Lightly oil a 23cm springform cake pan.
2.Heat oil in a large heavy-based frying pan over high heat; cook onion, stirring, for 3 minutes or until softened. Add spinach leaves; cook, stirring, for 4 minutes or until just wilted. Season well. Cool slightly.
3.Beat eggs in a medium bowl until combined; stir in spinach mixture, fetta and dill. Mash with a fork to combine.
4.Place 1 sheet of fillo pastry on a clean surface, lightly brush with a little melted butter; top with another sheet of fillo. Repeat layering with another 2 fillo sheets, brushing between each sheet with butter. Ease fillo stack into springform pan, allowing edges to overhang. Repeat layering and brushing process with another 4 fillo sheets. Place second fillo stack in pan in the opposite direction, allowing edges to overhang.
5.Carefully pour spinach mixture into pastry case. Brush any overhanging fillo with butter; neatly fold overhanging fillo over spinach mixture to enclose. Repeat layering process with remaining 4 fillo sheets, brushing with melted butter; scrunch the stack and place on top of the pie. Sprinkle with pepitas.
6.Bake pie for 45 minutes or until golden. Cool in pan for 5 minutes; transfer to a board. Cut pie into wedges, sprinkle with extra dill.
Cover fillo with a clean tea towel to prevent it from drying out while layering, pulling out only the sheets that you need at each stage.
Refrigerate any leftover pie in an airtight container for up to 3 days.