1. Preheat oven to 400 degrees with rack in bottom third. Hear 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 T thyme, then gradually whisk in milk. Simmer bechamel 2 minutes, whisking. Remove from heat, and add 1 tsp salt and 1/2 tsp pepper
2. 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.
3. Remove from oven. Preheat broiler. Spread bechamel on top, and sprinkle with remaining cheese. Broil until golden brown and bubbling, 6-7 minutes. Remove from oven; garnish with pepper and remaining thyme. Let sit 10 minutes before cutting.
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