Croque Monsieur

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.