1 tbsp butter, plus softened butter for bread
1 tbsp flour
3/4 cup whole milk
Pinch of grated nutmeg
Coarse salt and ground black pepper
2 tbsp Parmigiano, grated
3 oz Gruyère cheese, grated (about 3/4 cup). If you can, get Franche-Comté.
4 slices French or Italian loaf bread
Dijon mustard
6 oz thinly sliced ham, about 6 slices. (We used a package of Applegate Farms organic uncured ham: 7 oz equals 7 slices.)