- 1 1/4 cups whole milk
- 2 tablespoons unsalted butter
- 3 tablespoons all-purpose flour
- 3 tablespoons fresh thyme leaves, divided
- Coarse salt and freshly ground pepper
- 1 long loaf rustic bread, such as ciabatta (about 1 1/2 pounds)
- 3 tablespoons Dijon mustard
- 3/4 pound sliced ham
- 4 cups grated Gruyere cheese (about 8 ounces)
- 1 cup finely grated Parmesan cheese (1 ounce)
- Preheat oven to 400 degrees with rack in bottom third. Heat milk in a saucepan over medium heat until hot, about 5 minutes. Melt butter in another saucepan over medium heat; add flour, whisk to combine, and cook 2 minutes, stirring occasionally. Add 1 1/2 tablespoons thyme, then gradually whisk in milk. Simmer bechamel 2 minutes, whisking. Remove from heat, and add 1 teaspoon salt and 1/2 teaspoon pepper.
- Cut top crust off bread; slice loaf in half horizontally. Fill sandwich with mustard, ham, and half the Gruyere. Toast on a baking sheet until cheese is melted and top is golden brown.
- Remove from oven. Preheat broiler. Spread bechamel on top, and sprinkle with remaining cheese. Broil until golden brown and bubbling, 6 to 7 minutes. Remove from oven; garnish with pepper and remaining thyme. Let sit 10 minutes before cutting.