Beat the egg with a fork, crumble the feta cheese, grate the cheese coarsely. Open the packet of puff pastry. Smear it with the egg.
Distribute cheese and feta cheese on top evenly, all over the entire puff pastry, and wrap it into a roll.
With a sharp knife, cut the roll into pieces 1/2″ (1.5 cm) thick and arrange in a tray.
Sprinkle each piece with poppy seeds. Bake in a hot oven until the puff pastry reddens.