Melt the butter in a saucepan over medium heat. Allow it to bubble slightly before whisking in the flour, cooking for about two minutes until it forms a smooth paste.

Gradually add the whole milk while continuously whisking. Keep stirring until the béchamel sauce thickens and becomes creamy, which should take around 5-7 minutes.

Stir in the dijon mustard, a sprinkle of nutmeg, and season with salt and pepper to taste.

Preheat your oven to 400°F if baking, or heat up a skillet on medium-high if grilling.

Slice the baguette into small pieces, then layer ham and gruyère cheese between two slices.

Generously pour the creamy béchamel sauce over each sandwich, ensuring coverage. Add extra gruyère on top for an even cheesier bite.

Place the assembled mini sandwiches in the oven or skillet. Cook until the top is golden brown and bubbly, about 10-15 minutes.