In a medium saucepan over medium heat, melt 4 tablespoons of unsalted butter until bubbly. Whisk in ¼ cup of flour, stirring constantly for about 4–5 minutes until golden. Gradually add 2 cups of cold whole milk, whisking until smooth. Toss in a bay leaf and continue to cook for another 5 minutes until the sauce thickens. Stir in 2 tablespoons of whole-grain mustard, a pinch of freshly grated nutmeg, and salt to taste.

Preheat your oven to 400°F (200°C). Take slices of country-style French bread and spread a generous layer of the creamy béchamel on one side of each slice. On half the slices, layer with a slice of Gruyère cheese and 2 ounces of deli ham. Top with the remaining bread slices, béchamel side facing up, and add more Gruyère on top.

Place the assembled sandwiches on a baking sheet lined with parchment paper. Bake in the preheated oven for about 15 minutes, or until they become golden and bubbling.

Once baked, switch your oven to broil for about 1 minute, watching closely to prevent burning. The cheese should be bubbly and slightly crisped.

Remove the sandwiches carefully from the oven and let them cool for a moment. Slice each Croque Monsieur in half and serve warm with a side of salad or cornichons.