Preheat the oven to 400° F
In a large frying pan over medium heat, heat the olive oil.
Add the onion and sauté for 8 -10 minutes, without browning.
Add the chard stems and the celery and continue cooking for 4 minutes, stirring occasionally.
Increase the heat to medium-high, and add the chard leaves, stirring as they cook for another 4 or 5 minutes, or until the leaves have wilted
Add the scallion/green onion, arugula, and herbs. Cook for an additional 2 minutes or so. Remove from the heat and transfer the mix to a colander to cool.
Once the mixture is cool, squeeze out as much water as you can and transfer to a mixing bowl. (I've found a Go to Amazon - Nut Bag works the best when it comes to squeezing out excess water)
Add the three kinds of cheese, lemon zest, eggs, salt, pepper. Mix well
Lay out a sheet of phyllo pastry and brush it with some olive oil. Cover with another sheet and continue in the same manner until you have 5 layers of filo brushed with oil, all covering an area large enough to line the sides and bottom of an 8 ½" pie dish, plus extra to hang over the rim. Line the pie dish with the pastry, fill with the herb mix and fold the excess pastry over the edge of the filling, trimming the pastry as necessary to create a ¾" border.
Make another set of 5 layers of phyllo brushed with oil and place them over the pie. Scrunch the pastry a little to create a wavy, uneven top and trim the edges so it just covers the pie.
Brush generously with olive oil
Cover with aluminum foil and bake for 20 minutes
Uncover the dish and bake for another 20 minutes, or until the filo has turned golden brown. (In the original recipe the dish is baked uncovered for 40 minutes. I found it browns more than I'd prefer, therefore the step of covering it for part of the baking time)
Remove from the oven and serve warm, or at room temperature.