1. Heat oil in a large saucepan or skillet. Add the onions and cook, stirring frequently, until transluscent.
2. Add the spinach and a bit of salt and pepper, cover, and cook until the spinach is completely wilted, stirring once or twice to hasten the process.
3. Scrape the spinach into a colander and let cool completely. Once cool, firmly squeeze out the excess liquid then chop the spinach with a chef’s knife into smallish pieces.
4. Mix the spinach in a small bowl with the feta and parsley until chunky. Taste, and add nutmeg and a squirt of lemon juice, plus more salt and pepper if desired. Stir in the egg.
5. Unwrap and unroll the filo and keep it covered at all times with a damp tea towel.
6. Working quickly lay one sheet of filo on the counter and brush it lightly, but thoroughly, with butter. Lay another sheet on top of it and brush it with butter as well.
7. Set a scant 1/4 cup (50g) of the filling in the center, about 1-inch (3cm) from the edge of the sheets of filo, then roll the two edges of the dough over, lengthwise, to encase the filling. You should have a long rectangle with filling underneath the top far end.
8. Brush the exposed surface of the filo with butter and fold one corner diagonally over the filling, then continue folding keeping the triangle shape (as you’d fold a flag) and brushing the exposed surfaces of the filo with butter, until you have a neat triangle. Brush the top with butter and set on a baking sheet in the freezer.
9. Continue making more spanakopitas with the remaining filling. Once all the spanakopitas are frozen, store them in a freezer bag until ready to bake. If well-wrapped, they’ll keep for a couple of months.
10. To bake the frozen spanakopita, preheat the oven to 350F (180C) and put the frozen triangles on a baking sheet, then brush each with butter. Bake for 30 minutes, or until deeply-golden brown. If you’re baking them without freezing them first, they’ll take less time to bake, so check them before the recommended baking time.