Toast the bread until lightly browned on one side.
Meanwhile, in a small saucepan over medium heat, melt the butter. Add the flour and whisk until thick and bubbly. Gradually whisk in the milk, until the sauce is smooth and thickened. Whisk in the paprika. Season to taste with salt and pepper. Keep the sauce warm while your assemble the sandwiches.
Lay four slices of bread, toasted side down on a broiler pan. Brush each one with Dijon mustard, to taste. Lay one slice of ham on each piece . Divide ½ cup cheese among the four sandwiches. Top with the second slice of bread, toastier side up. Divide the sauce among the four sandwiches, spreading it over the top. Sprinkle the remaining cheese over the sandwiches.
Broil until the cheese is melted and the tops are bubbly and brown, five minutes or more in my toaster oven. Watch it carefully; yours may be different. Serve immediately.
Notes
Turn this into a croque-madame and serve with fried egg on top!