Place 2 tablespoons olive oil into a large, deep frying pan over medium heat. Add the onions and sauté until softened but not browned, about 8 minutes. Add the spinach stems and celery. Cook for a further 6-8 minutes, stirring occasionally until softened. Depending on the size of your frypan, you may need to transfer this mixture into a bowl and cook the spinach leaves separately.

Add spinach leaves and stir as you cook, for about 5 minutes until the leaves wilt. Add rocket, parsley and mint, and return the onion/celery mixture to the pan. Cook for 5 minutes. Remove from heat and transfer to a colander to cool.

Preheat the oven to 200°C (400°F). Once the spinach is cool, transfer to a large mixing bowl. Add the three cheeses, lemon zest and juice, eggs, salt and pepper to taste. Mix well.

Lay out a sheet of fill pastry and brush with 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.