Melt butter, add flour and mix the roux obtained. Pour cold milk little by little, mixing vigorously. Bring to a boil then lower the heat and cook 4 to 5 minutes, stirring. Let cool.
PREPARE croques monsieur
Preheat the oven in the grill position (about190 ° C / 375°F).
Place 4 bread slices on a oven baking tray covered with baking paper.
Cover each loaf of bread with béchamel sauce.
Cover with grated cheese.
Add a slice of white ham cut into pieces, 1 per croque Monsieur.
Place the second slice of bread on top.
Cover again with béchamel.
Finally add more grated cheese in a generous layer.
Cook under the grill for ten minutes until croque Monsieur is hot, cheese melt and grilled.