Combine the shredded cheese, feta and egg yolk in a bowl.
Roll out the pastry sheet on a lightly floured surface to a 10 x 12-inch (25 x 30 cm) rectangle and place on an uncreased baking sheet. Lightly beat the whole egg with the water and brush over pastry
Scatter cheese mixture over the pastry leaving a 1-inch/2.5 cm border on all sides.
Bake for 10 minutes in a preheated 425 F/220 C oven.. Top with chopped herb(s) and cut into quarters then cut the quarters into thin wedges.