Ham and cheese is a classic sandwich combination. We give this panini a slight French flair by adding Dijon mustard and cheese sauce (Mornay), which melt into a perfectly oozy filling.
To make the sauce: Melt the butter in a saucepan set over medium-high heat.
Add the flour, stirring to incorporate. Slowly pour in the milk, stirring constantly, until a thick sauce forms.
Bring to a gentle simmer then add the cheese, stirring until it melts.
To make the sandwiches: Spread the warm sauce on one side of half the bread slices.
Top the sauce with a slice of ham, then a slice of cheese.
Spread the Dijon mustard on one side of the remaining slices of bread, and place them mustard-side-down onto the cheese.
Heat an electric panini griddle or skillet over medium-high heat.
Spread the top of the sandwich with butter, and place it butter-side down onto the hot surface.
Spread butter on the other side of the bread, then close the griddle or cover the pan with a lid.
Toast the sandwich for 3 to 5 minutes on one side, flip it over, and toast for another 3 to 5 minutes until it's as golden as you like. Remove from the pan, and serve warm.
