In a large bowl, combine the well-drained spinach (I squeeze it in a sieve and then in some paper towels to get all the excess water out), parsley, dill, chives, garlic, and green onions together.
Add the lemon juice, olive oil, eggs, feta cheese, Greek yogurt, salt, and pepper. Stir until thoroughly combined.
Brush the bottom and sides of a 9x13 pan with melted butter and add two sheets of fillo dough, overlapping so the bottom and sides of the pan are covered. Continue to brush the layers with butter, adding more fillo dough until you have about 12-15 layers of dough.
Spread the filling evenly in the pan and add 2 sheets of fillo dough on top and brush with butter. Repeat with the remaining sheets, about 12-15, layering 2 sheets at a time and brushing with butter. Try to save 2 sheets that have not ripped for the top layer. Brush with butter and bake for 50-60 minutes, until the fillo dough is golden brown. Serve warm or chilled.