Add the butter to a heavy bottomed saucepan over medium-low heat. Once the butter is melted and starts to bubble add the flour whisking continually until it starts to form a blonde color paste (not brown) and starts to bubble - about 2-3 minutes.
Slowly, pour in the milk, whisking until completely combined and the sauce has thickened. Then add the nutmeg, salt, and pepper. Gently bring to a low boil, stirring frequently to prevent burning and then lower heat. Cook for 2 to 3 minutes more and then remove from heat completely.
Stir in the Parmigiano cheese. Taste the sauce for seasoning and add more salt, pepper if you like. Set aside.
Preheat the oven to 400 degrees. Brush bread slices on one side with melted butter. Place on a baking sheet ( butter side facing up) and bake until lightly toasted and golden brown. Remove from oven.
Turn the bread slices over (butter side down). Add a few slices of the spiral ham to four of the slices, and sprinkle each with 4 tablespoons of the Gruyere. Spread the remaining slices with dijon mustard and flip on top of the ham and cheese side making the sandwich.
Spoon bechamel on top of each sandwich. Sprinkle each with 2 tablespoons Gruyere and with a pinch of the herbes de Provence. Bake for 10-12 minutes or until topping is golden and lightly browned.
Slice in half and serve with dressed mixed greens and cornichons (mini French pickles). Enjoy your creamy, cheesy sandwich!