01 - Set the oven to 400°F (200°C) to reach the desired baking temperature.
02 - Line a 9-inch pie dish with the thawed puff pastry, trimming and pressing edges into the sides. Prick the base with a fork and chill while preparing the filling.
03 - Heat olive oil in a skillet over medium heat. Cook the chopped onion until translucent, approximately 4 minutes, then add minced garlic and sauté for an additional 1 minute.
04 - Add fresh or thawed spinach to the skillet and cook until wilted or heated through. Drain the mixture in a colander, pressing out excess moisture, then allow to cool slightly.
05 - In a large bowl, whisk eggs thoroughly. Incorporate feta, ricotta, Parmesan, nutmeg, salt, pepper, and optional dill. Fold in the cooled spinach mixture until uniformly blended.
06 - Pour the prepared filling into the chilled pastry-lined dish, smoothing the surface. Brush the pastry edges with beaten egg to promote browning.
07 - Bake in the preheated oven for 35 to 40 minutes until the top is golden and the filling is firmly set.
08 - Allow the pie to rest for 10 minutes after baking before slicing for optimal texture and flavor.