Preheat your oven to 200°C (392°F).
Spread butter on one side of each slice of bread.
On the unbuttered side, spread Dijon mustard on two slices.
Layer ham and a generous amount of Gruyère cheese on the mustard side.
Top with the other slices of bread, buttered side facing out.
In a saucepan, melt 25g of butter over medium heat.
Add flour and stir for 1-2 minutes to create a roux.
Gradually whisk in the milk, stirring until thickened.
Season with salt and pepper, then remove from heat.
Pour the béchamel sauce over the sandwiches and sprinkle the remaining cheese on top.
Bake in the oven for 10-12 minutes or until golden brown.
Garnish with fresh parsley before serving.