Prepare the Filling:
If using fresh spinach, wash it thoroughly. Heat a little olive oil in a large skillet. Add the chopped onion and cook until softened. Add the minced garlic and cook for another minute.
Add the spinach to the skillet and cook until wilted. If using frozen spinach, ensure it’s well squeezed dry before adding it to the skillet.
Remove from heat and let it cool slightly.
In a bowl, combine the cooked spinach mixture, crumbled feta cheese, beaten egg, Parmesan cheese (if using), salt, pepper, and nutmeg (if using). Mix well.

Assemble the Rolls:
Preheat your oven to 200°C (400°F). Line a baking sheet with parchment paper.
Unfold the thawed puff pastry on a lightly floured surface. If you have two sheets, then work with one at a time.
Slightly roll out the pastry to make it a bit thinner.
Spread the spinach and feta filling evenly over the puff pastry, leaving a small border around the edges.
Starting from one long edge, carefully roll up the pastry like a log.
Cut the roll into individual slices, about 2-3cm thick.
Place the rolls on the prepared baking sheet.
Brush the rolls with the beaten egg wash.
Sprinkle with sesame seeds or poppy seeds, if desired.

Bake:
Bake in the preheated oven for 20-25 minutes, or until golden brown and puffed.
Let them cool slightly before serving.
Tips and Variations:

Add herbs: Fresh herbs like dill, parsley, or oregano add a lovely flavor to the filling.
Spice it up: A pinch of red pepper flakes can give the filling a nice kick.
Cheese variations: You can use ricotta cheese or cream cheese in addition to or instead of feta.
Sun-dried tomatoes: Add chopped sun-dried tomatoes for a Mediterranean twist.
Make it ahead: You can prepare the filling and assemble the rolls ahead of time and bake them just before serving.
Individual rolls: Instead of rolling the whole pastry, you can cut it into squares, fill each square, and fold them into triangles or rectangles.
Enjoy your delicious spinach and feta rolls!