Preheat the oven to 355°F (180°C).
Heat the olive oil in a large pan over medium heat. Sauté the chopped onion until it becomes translucent. Add the fresh spinach and chopped spring onions and cook until just wilted, about 1-2 minutes. Drain and squeeze out any excess moisture from the spinach mixture. Set aside to cool.
Combine the egg, feta cheese, salt, and pepper in a large mixing bowl. Once the spinach mixture has cooled, add it to the bowl and mix well.
Lightly grease a large round cake pan. Layer 12-15 sheets of phyllo dough in the pan, brushing each sheet with olive oil as you layer.
Spread the spinach mixture evenly over the layered phyllo dough.
Layer another 14-15 sheets of phyllo dough on top of the spinach mixture, brushing each sheet with olive oil. Ensure the spinach mixture is fully covered until you can't see the spinach mixture. Tuck in the edges of the phyllo to seal the filling.
Using a sharp knife, slice the spanakopita into squares or diamond shapes.
Bake in the preheated oven for 40-45 minutes, until the phyllo dough is golden brown and crispy.
Allow the spanakopita to cool slightly before serving.
Savor your delicious and authentic homemade Greek spanakopita.