Warm the milk in the microwave
In a saucepan, melt the butter over medium-low heat
When the butter is melted, add the flour and whisk until incorporated and the flour is cooked (about 45 seconds to 1 minute)
Add the warm milk and whisk vigorously to avoid lumps
Keep whisking until the sauce starts getting thick and creamy.
When it boils, remove from heat and add the dijon mustard, 1/4 cup of gruyere cheese, salt, pepper, and nutmeg. Mix until combined
Meanwhile, toast the bread slices slightly in the oven or toaster
Place the toasted bread slices on a sheet pan or a grill pan
Add the ham to 4 of slices of bread
Top the ham with grated gruyere cheese
Place the remaining bread slices over the gruyere cheese to make the sandwich
Top each of the 4 sandwiches with the warm bechamel sauce and extra gruyere cheese
Broil for 3-4 minutes or until the top is bubbly and toasted
Remove from oven, slice and enjoy!