Preheat oven to 180C or 170C fan forced.

Sauté the onion over a medium heat until translucent. Allow to cool. Combine spinach, cheeses, beaten egg, salt and pepper and cooled cooked onion in a bowl. Cut the pastry sheets in two. Spoon the spinach and cheese mixture into the middle of the pastry, leaving approximately 1cm on all sides. Place another sheet of thawed pastry on top of the spinach mixture and use your fingers to press down and seal the edges around the spinach, removing any big air bubbles. Use a form to crimp the edges, sealing the pastry further. Brush the top with egg wash and lightly sprinkle with sesame seeds. Slash the rolls with a sharp knife to allow steam to escape while baking. Bake the spinach buff pastry rolls for 18-22 minutes until they are golden and puffy.