Place a large skillet over medium heat. Add in the olive oil and chopped onion. Saute for about 8-10 minutes until slightly colored.
Meanwhile, wash the spinach or greens, shake off the water and chop it. Remove the onion from the pan and place in a large bowl. Saute the chopped spinach for 1-3 minutes until wilted. Place in a colander or wire mesh strainer. Allow to cool slightly and then squeeze out the excess water.
If using frozen spinach, simply defrost and then squeeze out the excess liquid.
Add the spinach to the bowl. Toss in the feta, herbs, salt, pepper and lemon juice. Taste it. Add more of any of the ingredients to your desired taste. Fold in the egg.
Preheat the oven to 375.
In a 9×9 casserole dish, brush or drizzle in a bit of the butter. Carefully, unfold the filo dough. Peel off 1-2 sheets of filo and place in the lightly buttered dish. Drizzle or brush butter on the filo and then cover with another sheet. If the filo tears apart simply glue it on to the other layers with a bit of butter. Once there are about 4 sheets with a light layer of butter in between dump in the spinach-feta mixture.
Peel off several more sheets of filo dough, place on top, drizzle the butter in between each layer. When you are not peeling apart the filo Dough, cover with a piece of plastic wrap and a slightly moistened (not wet) dish towel on top.
Cover with foil. Bake for 25-35 minutes until baked through and hot in the center. Remove the foil and bake for 5 more minutes until the filo is golden. Serve cut into pieces. If you want a traditional look, cut in diagonals to create triangles.
For leftovers, just warm in an oven at 350 covered with foil or serve at room temperature.