• Preheat oven to 400°F.

Make the Béchamel Sauce

• In a small saucepan, melt butter on medium/low heat until it just starts to bubble.
• Add the flour, stirring until smooth for about 2 minutes.
• Slowly add the milk, whisking continuously. Cook until it's thickened and then remove from heat.
• Add salt, pepper, and nutmeg and then slowly stir in the parmesan and about 1/4 cup of the Gruyère and stir to melt all of the cheese.

Assemble the Sandwiches

• Line a baking sheet with parchment paper. Add sliced bread and add them to the oven, cook for a couple of minutes on each side to toast. Or use a toaster oven - but you will still need parchment paper lined baking sheets.
• Add bread slices to parchment lined baking sheet and brush the toasted slices with mustard, to taste (we love a lot of mustard!). Add the ham slices and about 1 cup of the remaining Gruyère cheese. Top with the other toasted bread slices.
• Spoon on the béchamel sauce to the tops of the sandwiches. Sprinkle with the remaining Gruyère cheese.
• Bake in the oven on the medium rack for 5 minutes, then turn on the broiler and broil for an additional 3 to 5 minutes, until the cheese topping is bubbly and lightly browned. You can also grill the sandwiches in a skillet on the stove top and then just finish them in oven to melt the cheese.
• Slice in half and serve immediately.