Croque Monsieur Recipe

Warm the milk

Gently warm the 1 cup of whole milk in a small saucepan or in the microwave until hot but not boiling (about 1–2 minutes). Warming the milk helps avoid lumps in the sauce.

Make roux

In a medium saucepan over medium heat, melt 3 tablespoons unsalted butter. Add 3 tablespoons all-purpose flour and whisk continuously for 1–2 minutes until the mixture is bubbling and lightly golden (this cooks the flour).

Add milk

Slowly pour the warmed milk into the roux in a steady stream while whisking vigorously to prevent lumps. Continue whisking until the sauce thickens, about 2–4 minutes.

Season and finish

Reduce heat to low. Stir in 0.5 teaspoon Dijon mustard (optional), 0.5 teaspoon salt, 0.25 teaspoon freshly ground black pepper, and 1 teaspoon freshly grated nutmeg. Remove from heat and stir in 1/2 cup grated Gruyère until melted and incorporated. Keep the béchamel warm; if it thickens too much, whisk in a tablespoon of warm milk to loosen.

Prepare bread

Preheat the broiler (or oven to 425°F/220°C). Lightly butter one side of each of the 8 bread slices with the 2 tablespoons softened butter; these will be the outside of the sandwiches. Place the slices butter-side-down on a work surface.

Spread mustard

Spread 2 teaspoons Dijon mustard evenly on the unbuttered side of 4 bread slices (these will be the bottom slices).

Assemble sandwiches

On each mustard-spread slice, layer 2 slices of ham and then about 3 tablespoons (roughly 1/4 cup) of the grated Gruyère (from the 1.5 cups called for). Top with the remaining 4 bread slices, butter-side-up, forming 4 sandwiches.

Top with béchamel

Place assembled sandwiches on a rimmed baking sheet. Spoon a generous couple of tablespoons of warm béchamel sauce onto the top of each sandwich, spreading it to cover most of the top bread slice. Sprinkle the reserved 1/2 cup grated Gruyère evenly over the béchamel on each sandwich. Add a little freshly ground black pepper to taste.

Broil until golden

Place the baking sheet under the preheated broiler about 4–6 inches from the heat. Broil for 3–6 minutes, watching carefully, until the béchamel bubbles and the cheese is golden brown and slightly crisp. Rotate the pan as needed for even browning.

Optional pan finish

Alternatively, for a crispier exterior, after broiling you can melt 1 tablespoon unsalted butter in a skillet over medium heat and brown each sandwich butter-side-down 30–60 seconds to crisp the bottom.

Rest and serve

Let sandwiches rest for 1–2 minutes, then transfer to plates. Cut each Croque Monsieur in half on the diagonal and serve immediately.