Remove the green part of the green onions, and reserve.
Using a good sized frying pan, in the olive oil, sweat (reminder not to use salt if later using myzithra cheese) the basil, oregano, garlic, and the chopped white part of the onions over medium low heat for a few minutes. If you are using dried parsley, add it to the sweat, otherwise do not add at this stage. (If you are substituting fresh herbs for dried, also do not add now.).
When the onions are soft and starting to fall apart, add the spinach and any fresh herbs. Simmer for several minutes at the same heat. (The purpose of this is to add the flavor of the herbs and aromatics to the spinach.).
Let that cool. This is very important. Commune with your family and/or friends, and forget about it for a while. Let the flavors meld, and accept that there's a reason for this. Preheating the oven to 375 depends on the oven, and how long the cooling process takes and how comfortable you are with your utility bills.
Once the spinach mixture has reached at least a lukewarm temperature, in a mixing bowl, or if of limited means the same pan if it's large enough, add the feta, and get your hand(s) in there and start working the product. Depending on the liquid consistency, this is where this recipe becomes an art and a set of guidelines rather than a structured explanation. With your hand on the mixture, you're going to know whether it is feeling tacky or not. You want something like bread dough ready to bake, as far as tactile feel goes.
You will need one egg for the inner part of the eventual product to "set" properly. Add that at this point in the process. That will give you a feel for how the rest of this will go. The reason you wanted it to cool was so that you could feel the temperature without burns, and to ensure that the egg did not cook in the spinach mixture.
The strongly suggested myzithra should be added now. If that is not available, or outside of price range, then parmesan and romano should be added now. (It's a matter of preference, in the end, as the myzithra was the "WOW", but it's still an amazing dish without it.) What you want is at least a half cup combined. If the secondary cheeses (remember the feta was already added) do not drop the liquid consistency to "tacky", add more. If it is dry at this point, add another egg and add more cheese to get it to "tacky". What ingredients, dry or fresh, you used prior to this stage have an effect now.
Once you reach that proper consistency, wash your hands, you don't need to touch it any more (potential cross contamination issues with the eggs), and oil a baking dish of similar proportions to the filo dough you purchased. I suggest butter, but do not use olive oil. Its smoking point is too low, and will often burn at 375°F.
Lay out about half of the filo dough on the bottom of the oiled baking dish. Place the spinach mixture onto the lower layer of the filo dough. Make sure it reaches the edges. If there is some tearing, do not worry as long as the lower layers of the dough are still solid. This is normal, and will not comprise the final product. Lay the rest of the filo dough on top. Brush the top layer with the melted butter. This will assist in producing a crunchy top layer and help keep it from burning.
Place in preheated 375F oven. Cook for 10-15 minutes, and check. Apply more butter if needed. Cook for a total of 20-25 minutes. You should be able to smell it and hear a solid almost frying sound when it is ready. ?