To make the pastry, place the white and wholewheat flour, salt and butter in a food processor. Pulse it until the cubes of butter are the size of peas. Now add the water a tablespoon at a time and pulse between each addition. Tumble mixture onto a work surface and use your hands to just bring the pastry together. Shape a 5cm thick round disk and wrap the pastry in cling film. Place in the fridge for 30 minutes.
Make the filling while the pastry is resting. Parboil the potatoes until they just start to soften – about 10 minutes. Fry the leeks, garlic and sage leaves in the oil and butter over low heat until soft. Allow leeks to cool to room temperature.
Cut a piece of baking paper the size of a baking tray. Place the pastry in the middle of the paper and roll it out to be about 5mm thick. Slice the potatoes thinly and place on the pastry circle leaving a 4cm border clear. Next spoon the leeks onto the potatoes and spread out evenly. Fold the 4cm border over the filling. Place galette onto a baking tray and paint the pastry border with the egg wash. Grind over sea salt and plenty of black pepper. Pop galette into a pre-heated 180 degree Celsius oven. Bake for 25 minutes and only then top the leeks with Camembert slices. (If you add the Camembert right at the start of baking, it will disappear completely. This way it will hold its shape and be soft and oozy, just the way you want it!) Bake for a further 15-20 minutes until golden.
With all that butter and cheese this is a rich galette and I would not serve it as a main meal. Instead I would cut petite canapé-sized slices and serve them pipingly hot with an ice cold glass of lightly wooded, buttery Chardonnay as the sun sets.