In a medium saucepan over medium heat, melt 2 tablespoons of unsalted butter until frothy. Stir in 2 tablespoons of flour, cooking for about 4-5 minutes until golden and fragrant. Slowly whisk in 2 cups of cold whole milk, ensuring no lumps form, and continue heating until the mixture thickens, about 5 minutes. Remove from heat, then stir in 1 tablespoon each of whole-grain and Dijon mustard, a pinch of freshly grated nutmeg, and kosher salt to taste.

While the béchamel simmers, lay out 8 slices of country-style French bread on a clean surface. Use a spatula to generously spread béchamel on one side of each slice, ensuring complete coverage. Preheat your oven to 400°F (200°C).

On four of the prepared bread slices, layer thinly sliced deli ham and half of the grated Gruyère cheese. Top with the remaining slices of bread, béchamel side up. Sprinkle the remaining Gruyère over all sandwiches, along with a delicate dusting of herbes de Provence. Press down gently to ensure they hold together during cooking.

Place the assembled Croque Monsieur sandwiches on a baking sheet lined with parchment paper. Bake in your preheated oven for 15 minutes, or until the cheese is bubbly and golden brown. If desired, broil for an additional 1 minute for that perfect crispy finish.

Once baked to perfection, remove the Croque Monsieur sandwiches from the oven and allow them to cool for a minute. Carefully slice each sandwich in half, revealing the gooey layers within. Serve warm and enjoy.