Preheat your oven to 375°F (190°C).
In a mixing bowl, heat olive oil in a skillet over medium heat. Add chopped onion and sauté until soft, about 5 minutes.
Add chopped spinach to the skillet and cook for another 2-3 minutes until wilted. Remove from heat and let it cool slightly.
In the same bowl, combine ricotta cheese, feta cheese, eggs, oregano, salt, pepper, and nutmeg. Mix well.
Incorporate the sautéed spinach and onion into the cheese mixture until evenly combined.
Roll out one sheet of phyllo pastry and place it in the pie dish, allowing the edges to hang over the sides. Brush with a little olive oil.
Layer the second sheet of phyllo on top and brush with olive oil again.
Pour the spinach filling into the crust and spread evenly. Fold over the hanging edges of the phyllo over the filling for a rustic look.
Sprinkle grated Parmesan cheese on top.
Bake in the preheated oven for 30-40 minutes, or until the pastry is golden brown and the filling is set.
Remove from the oven and let cool for 10 minutes before slicing and serving.