Place your flour in a bowl and add your salt and 100g butter. Rub in with your fingers until the mixture resembles breadcrumbs and there are no visible streaks of butter.
Add your remaining butter and mix to combine before adding your ice cold water and quickly combining with a table knife. Bring together in the bowl briefly, then wrap in clingfilm and chill as a disc in the fridge for 1 hour.
Melt your butter in a saucepan before adding your flour. Cook out for 2 mins, then gradually beat in your milk until you have a smooth sauce. Bring to a boil and turn the heat down to a simmer until your sauce thickens. Season with salt and pepper, then add your cheese.
Place your chilled pastry in between two squares of parchment paper. Roll out into a large disc, then remove your top layer of greaseproof.
In the centre of your pastry, arrange your ham, leaving a 1 1/2 inch border. Top with your béchamel and cheese, then fold in the sides of your pastry bit by bit until you have created a firm border around your tart. Brush the edges with milk, then chill in the freezer for 15 mins, or until firm.
Heat your oven to 200°C and place a large baking tray inside to pre-heat.
Once your pastry is firm, gently slide the galette on its greaseproof paper onto your preheated tray. Bake in the oven for 35-40 mins, or until your pastry is cooked and golden.