Heat the butter in a pan. Add the flour, cook briefly; the flour must not take on any colour. Pour in the milk all at once, bring to the boil while stirring, reduce the heat and simmer for approx. 5 mins., season and allow to cool slightly.
Spread half of the bechamel sauce over 4 pieces of toast bread. Top each piece of bread with 2 slices of ham, sprinkle with half of the cheese. Place the remaining pieces of toast bread on top, press down gently, transfer to a baking tray lined with baking paper. Top with the remainder of the bechamel sauce and cheese, then season.
Approx. 10 mins. in the centre of an oven preheated to 220°C. Remove from the oven, cut the sandwiches in half, garnish with cress and serve hot.