Preheat oven to 400 degrees with rack in bottom third.
For the béchamel
Heat milk in a saucepan over medium heat until hot, about 5 minutes. Alternatively you can microwave the milk for about 2 minutes until hot (this will save you a dirty dish, too!)
Melt butter in another saucepan over medium heat; add flour, whisk to combine, and cook 2 minutes, stirring occasionally. Add 1½ tablespoons thyme, then gradually whisk in milk. Simmer béchamel 2 minutes, whisking.
Remove from heat, and add 1 teaspoon salt and ½ teaspoon pepper.
Cut top crust off bread; slice loaf in half horizontally. Spread mustard on one slice of bread. Fill sandwich with sliced ham and the Gruyere. Toast on a baking sheet until cheese is melted and top is golden brown.
Remove from oven. Preheat broiler. Spread béchamel on top. 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.
Amount Per Serving
Calories2233kcalCalories from fat 1300
% Daily Value
Percent Daily Values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs: