Croque Monsieur Gratiné with Prosciutto and Gruyère

Preheat your oven to 375°F (190°C). Lightly butter a baking dish or a small baking sheet.

Prepare the Béchamel: In a small saucepan over medium heat, melt 2 tablespoons of unsalted butter. Add 2 tablespoons of all-purpose flour and whisk continuously for 1 minute to create a roux. Gradually whisk in 1 cup of whole milk, ensuring no lumps form. Bring to a gentle simmer, whisking, until the sauce thickens to coat the back of a spoon, about 3-5 minutes. Remove from heat and stir in 1 teaspoon of Dijon mustard, 1/8 teaspoon of freshly grated nutmeg, 1/4 teaspoon of sea salt, and a pinch of freshly ground black pepper.

Assemble the Sandwiches: Lightly toast the 4 slices of brioche bread, either in a toaster or a dry pan, until just golden. Spread a thin layer of béchamel on one side of each brioche slice. On two of the slices, layer half of the 60 grams of thinly sliced prosciutto and half of the 1 cup of grated Gruyère cheese. Place the remaining two brioche slices on top, béchamel-side down, forming two sandwiches. Spread the remaining béchamel generously over the tops of the assembled sandwiches, ensuring full coverage, then sprinkle with the remaining Gruyère cheese.

Bake and Serve: Carefully transfer the sandwiches to the prepared baking dish. Bake in the preheated oven for 12-15 minutes, or until the cheese is melted, bubbly, and deeply golden brown. For an extra crisp topping, you may finish under the broiler for 1-2 minutes, watching carefully to prevent burning. Let rest for 2 minutes before serving hot.