Preheat the oven to 375°F (190°C) and lightly grease a 9-inch (23 cm) pie dish.
Heat the olive oil in a large skillet over medium heat. Sauté the onion for 4–5 minutes until softened, then add the garlic and cook for 30 seconds. Add the spinach and cook until wilted, then remove from heat and let cool slightly.
In a large bowl, combine the cooked spinach mixture, cream cheese, feta cheese, mozzarella, Parmesan, eggs, oregano, dill, garlic powder, onion powder, salt, black pepper, and nutmeg (if using). Mix until well combined.
Press one pie crust into the prepared pie dish, spoon the spinach filling evenly into the crust, cover with the second pie crust, trim and crimp the edges to seal, and cut 4–5 small slits in the top crust to allow steam to escape. Brush the top with the beaten egg and sprinkle with sesame or nigella seeds if desired.
Bake for 40–45 minutes, or until the crust is deep golden brown and the filling is set. If the edges brown too quickly, cover them loosely with foil during the last 15 minutes of baking.
Let the pie rest for 10–15 minutes before slicing and serve warm with Greek salad, tomato cucumber salad, roasted potatoes, soup, or tzatziki sauce.
Notes
Refrigerator: Store covered for up to 4 days.
Freezer: Freeze baked or unbaked pie for up to 3 months.
Reheating: Bake at 350°F (175°C) for 15–20 minutes, or until heated through.