01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Heat olive oil in a large skillet over medium heat. Add the diced onion and sauté until translucent, approximately 3 minutes. Add minced garlic and cook for an additional minute.
03 - Add chopped spinach to the skillet and cook, stirring occasionally, until wilted and excess moisture evaporates, about 4 to 5 minutes. Remove from heat and allow to cool slightly.
04 - In a large bowl, mix ricotta, feta, Parmesan, egg, ground nutmeg, salt, and freshly ground black pepper. Incorporate the cooled spinach mixture until fully combined.
05 - On a lightly floured surface, roll the puff pastry into a rectangle roughly 12 by 16 inches in size.
06 - Spread the spinach and cheese filling evenly along one long side of the pastry, leaving a 1-inch border.
07 - Carefully roll the pastry over the filling, sealing the seam and ends securely. Place the strudel seam-side down onto the prepared baking sheet.
08 - Combine the egg yolk with milk or cream and brush this mixture over the surface of the strudel. Optionally, sprinkle with sesame or poppy seeds.
09 - Bake the strudel in the preheated oven for 30 to 35 minutes, until the crust is golden brown and crisp.
10 - Allow the strudel to rest for 5 minutes before slicing into thick portions. Serve warm.