Heat the oil in a large pan. Add the onion and chicken and cook, stirring, until the chicken is cooked through.
Over a medium heat, add the garlic and then half the spinach. Continue to stir. When that half is wilted, add the second half. Once this is cooked through, place in a colander or sieve over a bowl and allow to cool. Discard any liquid that drains off.
Once cool, place in a bowl and mix through the eggs, feta and nutmeg and season generously with salt and freshly ground black pepper.
Pre-heat over to 200C/180C if fan assisted. Melt the butter.
Take a circular oven-proof dish or springform tin, approximately 8 to 9 inches in diameter. Lay one sheet of filo across the dish, so that one side and the base is covered, and the rest hangs over the other side. Brush well with melted butter, then lay another sheet of filo in a similar way, at ninety degrees to the first. Repeat with the other two, so that there's an overhang in each of four directions. This will be used to cover the filling.
Place the filling in the dish, on top of the pastry. Bring one of the ends of the pastry over the filling. Brush with butter, fold another over and repeat until all four ends are covering the top of the pie. Brush the top with butter and pop in the oven for 20-30 minutes, until the top is golden brown.
Let sit for 5 minutes before serving, to prevent it breaking up.