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!