I used Amish Milk bread to make this open-faced sandwich the last time we had this. I'm making that again today with some changes and hopefully it will come out correctly this time as the dough had a different feel to it than the 2 previous times I made it. BTW, this is definitely a knife and fork sandwich whether it has a top on it or not. You can make a croque madame sandwich by adding a runny fried or poached egg on top after broiling.
2 Tbsp unsalted butter
3 Tbsp all-purpose flour
2 cups hot milk
1 tsp kosher salt
1/2 tsp freshly ground black pepper
12 ounces Gruyere, grated (about 5 cups)
1/2 cup freshly grated Parmesan
8 slices white sandwich bread, remove crusts if you wish OR 16 slices if you want your sandwiches topped
8 ounces (or more if you want) baked ham, sliced but not paper thin
Preheat the oven to 400 degrees F.
Over low heat, melt the butter in a small saucepan. Add all the flour at once, stirring with a whisk for about 2-3 minutes until smooth and flour is cooked. Do not allow to brown. Pour the hot milk slowly into the butter and flour mixture, then cook, constantly whisking, until the sauce has thickened. Remove the pan from the heat and add the salt, pepper, nutmeg, 1/2 cup grated Gruyere, and the Parmesan. Set aside.
Toast the bread slices lightly on each side.
Spread the Dijon mustard on the toasted bread slices, add ham slices to each, and sprinkle with half the remaining Gruyere. If you want, top with another piece of toasted bread. Pour the cheese sauce over the sandwich, sprinkle with the remaining Gruyere, and bake the sandwiches for 5 minutes. Turn on the broiler and broil for 3 to 5 minutes, or until the topping is bubbly and lightly browned. Serve hot.