Prepare ingredients: Take the phyllo dough out of the freezer to allow it time to defrost. Using a clean kitchen towel, squeeze all excess moisture out of spinach. In a large skillet heat the olive oil on medium heat.
Assemble the filling: In a large skillet heat the olive oil on medium heat. Add the onion and green onion and saute until soft. Add the spinach and and saute until the spinach is limp. Stir in the lemon juice, and remove off the heat. Let cool. If there is excess liquid, place the mixture in a colander and squeeze out the excess liquid.
Finish the filling: Cool the spinach mixture completely. Beat the eggs with the feta and nutmeg in a medium size bowl. Add the cheese mixture to the spinach mixture and mix until it’s all incorporated well together.
Preheat the oven: To 350 F degrees. Line 2 baking sheets with parchment paper. Set aside.
Prepare the dough: Unroll the phyllo dough and lay one sheet flat on your work surface. For the remaining phyllo dough, cover it with a damp towel, so it won’t dry out. Brush the sheet with melted butter, then sprinkle with a bit of oregano. Add one more sheet and brush with melted butter and sprinkle with oregano.
Fill the dough: Using a pizza cutter, cut the sheets lengthwise into 4 long pieces, about 2 inch strips. Place a heaping tablespoon of filling near 1 corner of the phyllo strip. Fold the end at an angle over the filling to form a triangle. Continue to fold the triangle along the strip until you reach the end, like folding a flag. Repeat this with the remaining phyllo and filling.
Finish the spanakopita: Place your spanakopita triangles on your baking sheets and brush with butter, then sprinkle the Parmesan cheese over them if you wish. Bake them for 30 minutes until the triangles are golden brown and crisp. Let cool slightly before serving.