Preheat the oven to 400°
In a sauce pan over medium heat, melt butter and add flour. Whisk and allow to cook until the mixture turns a light golden brown.
Slowly pour in milk and whisk constantly until the mixture has thickened enough to coat a spoon and stay separated when a finger is run through it. Season with nutmeg, mustard, salt and pepper to taste.
Lay bread sliced on a lined sheet pan. Add 2 heaping tbsps of the sauce to each slice and spread to the edges. Top 3 of the slices with ham and half of the cheeses. Top the ham and cheese with the other 3 slices of bread, sauce side down.
Spread 2 more tbsps of sauce onto each sandwich and top with remaining cheese.
Bake for 15-20 minutes or until the cheese is golden brown and bubbling. Cool 5 minutes and serve.
