For the béchamel sauce, melt the butter in a small saucepan over low heat.
Stir in the flour using a wooden spoon, then cook gently for 1 minute, still stirring.
Remove the pan from the heat and gradually whisk in the milk until smooth.
Return the pan to the heat and continue to cook, stirring constantly with the wooden spoon, until the sauce thickens.
Season with salt and pepper, then remove from the heat and leave until cold.
To make the sandwiches, spread a layer of the béchamel sauce over one side of 4 slices of the bread, then top each one with cheese, ham and another slice of bread.
Melt the butter in a large, nonstick frying pan until foaming and fry the sandwiches, two at a time, 2-3 minutes each side. Serve with a sprinkle of sea salt.
Recipe adapted from James Martin’s French Adventure: 80 Classic French Recipes