01 - Set the oven to 400°F and line a baking sheet with parchment paper.
02 - Heat olive oil in a large skillet over medium heat. Add diced onion and cook until translucent, about 3 minutes. Add minced garlic and cook for an additional 1 minute.
03 - Add chopped spinach to the skillet and cook until wilted and liquid evaporates, approximately 4–5 minutes. Remove from heat and allow to cool slightly.
04 - In a large mixing bowl, combine ricotta, feta, Parmesan cheese, whole egg, ground nutmeg, salt, and pepper. Incorporate the cooled spinach mixture until evenly blended.
05 - On a lightly floured surface, roll out puff pastry into a rectangle approximately 12 x 16 inches.
06 - Spread the spinach and cheese filling evenly along one long edge of the pastry, leaving a 1-inch border.
07 - Carefully roll up the pastry around the filling, sealing the seam and ends. Position the roll seam-side down on the prepared baking sheet.
08 - Combine egg yolk with milk or cream and brush over the pastry surface. Sprinkle with sesame or poppy seeds if desired.
09 - Bake in preheated oven for 30–35 minutes until golden brown and crisp. Remove and let rest for 5 minutes before slicing.
10 - Slice into thick pieces and serve warm.