Preheat the oven to 180°C (350°F/gas mark 4).
Heat 1 tablespoon of the oil in a pan over a medium heat and add the onion. Sauté for a couple of minutes, until softened but not browned. Stir in the spring onions and cook for another minute. Add the spinach a handful at a time until it has all wilted down. Tip into a colander and press out any excess liquid, then cool.
Crumble the feta cheese into a large bowl and lightly mash with a fork. Mix in the cooled spinach mixture, eggs, Parmesan, mint and nutmeg. Season to taste.
Unroll the filo pastry and brush a 20cm (8in) clip-sided or loose-based tin with a little of the oil. Lightly brush each sheet with a little of the oil and place in the tin, oiled side down, leaving the excess hanging over the edge. Rotate the tin a quarter turn after putting in each sheet.
Tip the filling into the tin and fold the excess pastry over the top one sheet at a time to give a ruffled effect. Brush any remaining oil on top.
Bake for about 45 minutes, until crisp and golden brown. Remove from the oven and leave to settle for 5 minutes.
Take the spinach and feta pie out of the tin and cut into slices. Arrange on plates with the cherry tomato, red onion and basil salad to serve.