Start by making a simple bechamel sauce. In a medium saucepan, melt the butter over a low heat. Once melted, whisk in the flour. Cook this mixture for about two minutes to create a roux while stirring frequently.

Gradually pour in the hot milk while whisking vigorously to avoid lumps. Keep stirring until the sauce thickens, which should take about five minutes. Remove it from heat.

Now, add the mustard, a dash of nutmeg, salt, and black pepper. This is where you awaken the flavors! Mix well.

Take your slices of bread and spread the bechamel sauce generously over one side. Layer the ham on top, followed by the shredded Gruyère cheese. If you opted for caramelized onions, now's the time to add them. Top with another slice of bread.

In a large skillet, melt a tablespoon of butter over medium heat. Place the assembled sandwiches in the skillet, cooking until golden brown, about three to four minutes on each side. You want that crispy exterior!

As soon as the sandwiches are done, transfer them to a baking tray. Top with any leftover bechamel sauce and sprinkle with additional Gruyère cheese. Pop them in a preheated oven at 200°C (390°F) for about five minutes or until the cheese is bubbly and golden.

Slice your croque monsieur in half and serve immediately. Each bite combines textures and flavors that sing together like a well-rehearsed choir.