Preheat oven to 200 degrees Celsius. Fry onions in butter until soft. Add the garlic and cook for a couple of minutes. Add spinach and cook until wilted.
To make the pie filling, drain and cool the spinach then tip into a bowl and mix in the nutmeg, feta and eggs.
Grease a 20-22 non stick baking dish. Add single sheets of filo, brushing with melted butter, and keep going with the rest of the sheets until all are used up. Let the excess pastry hang over the side of the dish.
Tip in the filling and fold over the excess pastry to cover. Spread top with melted butter and score with knife. Bake for 30 minutes until the filo is crisp and golden.