Parmesan cheese helps gives this French sandwich its salty flavor.

2 tablespoons unsalted butter, plus more for bread

2 tablespoons all-purpose flour

2 cups whole milk

1/2 cup grated Gruyere cheese, plus 8 slices

1/2 cup grated Parmesan cheese

8 thick slices rustic bread

12 slices ham

Dijon mustard, for bread

Melt butter in a saucepan over medium heat. Whisk in
flour. Add milk; bring to a simmer. Cook, whisking frequently, until sauce thickens, about 5 minutes. Remove from heat. Add grated cheeses. Whisk until melted and smooth.
Butter bread. Arrange half the slices, buttered side down,
on a rimmed baking sheet. Top each with 2 slices Gruyere
and 3 slices ham. Spread mustard onto unbuttered sides of remaining bread. Place on top of ham, buttered side up.
Preheat broiler. Heat a grill pan or skillet over medium
heat. Cook each sandwich until golden brown and cheese
melts, about 5 minutes per side. Return to baking sheet.
Spoon 1/3 cup sauce over top of each sandwich. Broil until
tops are bubbling and golden brown, about 3 minutes.