Preheat oven to 400 degrees F.
Line baking tray with foil
In a small sauce pan melt the butter, then add flour all at once. Stir together for about 2 minutes.
Gently pour in the hot milk and whisk until thickened. Add all the parmesan and 1/4 cup Gruyere.
Continue to whisk until well blended, add a pinch of salt and pepper. Whisk again. Set aside.
Lay 8 slices of bread on baking tray.
Toast in the oven for 5 minutes, flip over and toast for another 2 minutes.
On half of the bread slices spread on dijon mustard.
Add two pieces of ham and sprinkle with Gruyere cheese.
Top each sandwich with remaining slices of bread and on top generously spread on Bechemal sauce top side up.
Sprinkle with Gruyere.
Bake for 5 minutes and then turn on the broiler for 4-5 minutes until cheese is melted and bubbly.