Preheat the oven to 350F. In a large mixing bowl, beat together the eggs, salt and pepper. Set aside.
In a large skillet, heat the olive oil until over medium high heat until shimmering. Add in the chopped spinach and onion, and cook until the onion is translucent and most of the excess moisture from the spinach has been evaporated. Remove from heat and cool slightly.
Fold the cooked spinach mixture into the beaten eggs until well combined.
Grease a 9 inch baking dish or oven safe skillet with a little melted butter. Unfold the phyllo dough and gently remove one sheet of phyllo pastry from the stack. To prevent the stack of phyllo pastry from drying out, keep a damp tea towel over the top of it,
Lay the piece of phyllo down onto the baking pan, gently scrunching excess dough around the edges to form a slight ruffled shape.
Brush the phyllo with melted butter and repeat the same process of overlapping sheets of phyllo and brushing them with butter until the last phyllo sheet is used.
Pour in the egg mixture and gently stir the mixture to even out the filling.
Brush any remaining melted butter around the edges of the dish. Bake for 20-25 minutes or until the crust is golden brown and the egg mixture is no longer jiggly when you shake the pan.
Remove from the heat and cool for about 10-15 minutes before cutting.