Preheat your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add chopped onion and sauté until translucent, about 4-5 minutes. Add garlic and sauté for an additional minute.
In a mixing bowl, combine the drained spinach, ricotta cheese, crumbled feta cheese, 2 of the eggs, Parmesan cheese, salt, pepper, and nutmeg. Mix well until all ingredients are thoroughly combined.
Once the onion and garlic mixture is done, add it to the spinach mixture and stir until evenly distributed.
Roll out the pie crust and place it into the pie dish, trimming any excess crust hanging over the edges.
Pour the spinach filling into the prepared pie crust, spreading it out evenly.
In a small bowl, beat the remaining egg and brush it over the top of the pie for a nice golden finish.
Bake in the preheated oven for about 40 minutes, or until the pie is set and the top is golden brown.
Remove from the oven and let cool for 10 minutes before slicing and serving.