Melt 4 tablespoons of unsalted butter in a medium saucepan over medium heat until bubbling. Stir in ¼ cup of all-purpose flour and cook for about 2 minutes until it turns light golden. Gradually whisk in 2 cups of cold whole milk, stirring continuously until thickened and creamy. Remove from heat and whisk in 1 tablespoon of whole-grain mustard, 1 tablespoon of Dijon mustard, a pinch of freshly grated nutmeg, and salt to taste.

Preheat your oven to 400°F (200°C). Spread a generous layer of the béchamel sauce on one side of each of the 8 slices of country-style French bread, reserving some for the topping.

On four of the béchamel-covered slices, layer with 4 ounces of deli ham and half of the grated Gruyère cheese. Top with the remaining four slices, béchamel side facing up, and sprinkle with the remaining Gruyère cheese and a teaspoon of herbes de Provence.

Place the assembled Croque Monsieurs on a parchment-lined baking sheet and bake for about 15 minutes, or until the cheese is bubbly and the bread is golden brown.

For extra browning, switch your oven to broil and broil the sandwiches for an additional 1–2 minutes, watching closely to avoid burning.

Let the sandwiches cool for about 5 minutes before slicing in half diagonally. Serve warm, possibly with a light salad or cornichons.