Thaw frozen spinach and frozen phyllo dough overnight in the refrigerator before using. Do not leave it in the fridge for more than a day though.
Preheat conventional oven to 350°F or convection to 325°F.
Prep your ingredients: squeeze the thawed, chopped spinach until you've wrung out all the water. Remove stems from dill and chop the white ends off green onions/scallion. You can finely chop the green onion and dill OR throw them in your food processor and pulse a few times until they're combined and finely chopped (one of my favorite shortcuts because it saves so much time).
Next, mix your greens. Combine the chopped spinach, dill, and green onions in a mixing bowl and drizzle olive oil over it. Stir to make sure the olive oil is distributed. Set aside.
In another mixing bowl, whisk 5 eggs, cream cheese, cottage cheese, kefalotyri, sea salt flakes, celery salt, and dried mint in your stand mixer. Once those are mixed and eggs are no longer "streaky," add in the feta on the lowest speed setting. Once feta has been added, slowly incorporate your greens . Now the filling is ready to go!
Grease your baking sheet. Lay out thawed phyllo dough. Pull 2 sheets + brush with melted butter (cover unused phyllo with damp paper towel in between so it doesn't dry out). Using a pastry brush generously coat the 2 phyllo sheets in your baking sheet with melted butter. Once they are coated edge to edge, grab another 2 sheets of phyllo dough and repeat, Repeat one last time so that you have a total of 6 sheets of phyllo as your bottom layer in the baking sheet.
Gently scoop spinach and feta filling over the buttered phyllo. Evenly distribute it.
Now, layer another 3 sheets of phyllo, and brush with melted butter, After the first 3 sheets are well coated, layer another 3 sheets of phyllo, and melted butter. Last, place another 3 sheets of phyllo and brush all over with melted butter
Brush butter on all of the edges of the phyllo and cut each corner GENTLY roll up the edges of phyllo around the border of the baking sheet. This will make a nice edge crust. Brush melted butter over the top of the rolled edges.
Score the phyllo to precut it (it's harder to cut when it's crispy)
Bake until the top layer of phyllo turns golden brown. If you're using a conventional oven bake at 350°F (typically takes around 40 minutes but start watching it at 30 minutes). It usually around 45 minutes to bake so it's just right. If baking this in an oven with convection, make sure you lower the temperature and bake it at 325°F. Still be sure to check it at 30 minutes