Preheat the oven to 220°C / 425°F / Gas mark 7.
Mix the lemon zest with the oil and set aside for 5 minutes.
Heat half the lemon flavoured oil in a large saucepan.
Add the leeks, spring greens, petit pois, tarragon and mint.
Mix well to coat the vegetables, then cover and cook over a low heat for about 5 minutes, stirring occasionally, until the greens are lightly cooked.
Season the vegetables to taste with salt and pepper, then transfer them to a 20 cm pie dish.
Beat the eggs with well the yoghurt.
Add a little seasoning and the cheese then pour the mixture evenly over the vegetables and mix through.
Brush a sheet of filo pastry lightly with a little of the remaining lemon oil and lay it over the vegetables, tucking the edges inside the rim of the dish.
Brush the remaining sheets of filo with oil and place them on top of the pie, oiled side up, pinching evenly.
Lay a piece of kitchen foil loosely over the top of the pie, and bake for 10 minutes.
Remove the foil and bake for a further 10–15 minutes with the extra greens on top and until the pastry is crisp and golden brown.