Preheat the oven to 200°C.
Pour 2 tablespoons of olive oil into a large, deep frying pan over medium heat.
Add the spinach/kale stems and celery. Cook for 4 minutes, stirring occasionally until softened.
Add the leaves to the pan and keep stirring for another 4 minutes until the leaves wilt.
Add rocket, parsley and mint and cook for another 2 minutes.
Remove from heat and transfer to a colander to drain and cool (you'll be surprised how much liquid comes out of this mix!).
Once the spinach mixture has drained and cooled down, add the three cheeses, lemon zest and juice, eggs and salt and pepper into a large mixing bowl. Mix well.
Lay out a sheet of fill pastry and brush with some olive oil. Cover with another sheet and continue in the same manner until you have 5 layers.
Line the bottom and sides of a 22cm pie dish with enough pastry, allowing some to overhang the rim. Fill with spinach mixture.
Make another set of 5 filo layers brushed with oil and place them over the top of the pie. Fold over the overhanging edges to enclose.
Brush generously with olive oil and bake for 40 minutes, or until the pastry turns a nice golden brown.
Serve warm or at room temperature.