Defrost the puff pastry according to package directions.
Blanch the spinach in enough salted water and then immediately rinse in cold water, drain and drain well, then blot a little.
Season the spinach with salt, pepper and the crushed garlic and mix lightly.
Place the puff pastry on a lightly moistened baking sheet. Melt the butter.
Place a strip of spinach in the center of the pastry, spread some feta cheese evenly over it and drizzle a little butter over the top. Repeat the process until all the ingredients are used. Close the strudel and brush it with the beaten egg.
Bake the strudel in a preheated oven at 250 °C for 15 minutes and then finish at 180 °C for another 15 minutes. Cut into slices and serve either warm or cold.