Mix the flour, cheese, lemon zest, herbs and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Add the egg white, mix quickly to create a soft dough. Shape the dough into a roll approx. 4 cm in diameter, cover and chill for approx. 1 hr. Cut the dough into slices approx. 3 mm thick, place on two baking trays lined with baking paper.
Brush the sablés with water. Scatter the herbs over the top. Sprinkle with fleur de sel.
Bake each tray for approx. 10 mins. in the centre of an oven preheated to 200°C. Remove from the oven, allow to cool slightly, then leave to cool completely on a rack.