Heat the oven to 180 C. Melt the butter in a saucepan. Add flour, stir gradually until smooth for about 2 minutes. Pour in the milk gradually, continuing to stir, and keep on fire until a thick, slightly golden consistency. Remove from heat, add mozzarella, salt and pepper.
Lightly grease pieces of bread with butter and place them on a baking sheet. Send it to the oven for 3 minutes. Then remove from the oven, put the ham on the bread and about 30 gr. cheese in 3 pieces. Collect 3 Croc Monsieur sandwiches by combining the halves.
Top each Croc Monsieur sandwich with a sauce. Sprinkle with the rest of the grated cheese. Bake in the oven for about 5 minutes and then about 3 minutes under the broiler until brown crust and melted cheese.