Beat the egg with half the water and a large pinch of salt. Add the flour and mix to make the pastry. Add the rest of the water to make the pastry more fluid, and leave to rest in the fridge for 1 hour. Cook the galettes in a non-stick frying pan, like pancakes.
Leek cream: gently cook the sliced leeks in butter; add the crème fraîche, season and simmer.
Garnish a warm galette with the creamy leek mix topped with slices of Pont l’Evêque and 3 slices of ‘Andouille de Vire’ heated under the grill or quickly fried in a pan, and fold the galette in half to serve.