Pastry: rub in (or combine) the flour and the butter, either using your fingers or a foodmixer and add the salt.
One tablespoon at a time, add the cold water. Keep going until a smooth dough comes together. If you need more water, add a further ½ tablespoon at a time. You are aiming for a dough that sticks together well but isn't tacky to the touch. Cover and place in the fridge for 30 minutes.
Filling: melt the butter and add the sliced onion. Fry until softened. Add the garlic and fry for a further minute or two.
Add the Worcestershire sauce and cook for one more minute. Remove from the heat and stir in the Greek yoghurt. Leave to cool. Pre-heat the oven to 180°C.
Roll out the dough on baking paper. You want to aim for a thin, rough, fairly large, circle. Move the galette (along with the baking paper) on to a large baking tray.
Stir the cheese into the filling mixture, then, place the filling in the middle of the pastry circle, spreading it around evenly. Leave about 5cm around the edge of the whole circle.
Turn over the edges all the way around the circle and press the pastry gently down into the filling (not too hard, just so that it attaches a little). Beat the egg and wash over with a brush before sprinkling with sea salt.
Bake for around 35 minutes until the pastry is nice and golden. Allow to cool a little and enjoy warm or cold.