01 - Set the oven to 400°F and line a baking sheet with parchment paper.
02 - Heat olive oil in a skillet over medium heat. Sauté the onion for 3 minutes until softened, then add garlic and cook for 1 minute.
03 - Add chopped spinach to the skillet and cook until wilted, about 2 to 3 minutes. Remove from heat and let it cool slightly.
04 - In a large mixing bowl, combine ricotta, cheddar, parsley, oregano, nutmeg, salt, black pepper, and the cooled spinach mixture. Stir thoroughly to blend.
05 - On a lightly floured surface, roll out the puff pastry sheets and cut each sheet into four equal squares, totaling eight squares.
06 - Place two tablespoons of filling in the center of each square. Fold over to form triangles and press edges firmly with a fork to seal.
07 - Transfer the pastries to the prepared baking sheet and brush their tops evenly with the beaten egg.
08 - Bake in the preheated oven for 18 to 22 minutes until golden brown and puffed.
09 - Allow pastries to cool slightly before serving.