Step by step

Method

4 steps

1. 1

   Heat the oven broiler. Melt 1 tablespoon unsalted butter in a small saucepan over medium heat. Whisk in 2 tablespoons all-purpose flour and cook 1 full minute until it smells toasty. Gradually whisk in 1 cup whole milk, ⅛ teaspoon ground nutmeg, ¼ teaspoon kosher salt, and a grind of black pepper. Simmer 3–4 minutes until thick enough to coat a spoon — that's your béchamel.

2. 2

   Spread about 4 teaspoons Dijon mustard on 4 of 8 slices sturdy white sandwich bread. Layer on 8 thin slices ham and half of 1½ cups shredded Gruyère, then top with the remaining bread.

3. 3

   Melt the remaining 2 tablespoons butter in a skillet over medium heat. Toast the sandwiches on both sides until golden, 2–3 minutes per side.

4. 4

   Move the sandwiches to a sheet pan. Spread béchamel over the tops and sprinkle with the remaining Gruyère. Broil 2–3 minutes until the topping is puffed, bubbly, and golden. Rest a minute so you don't burn your mouth, then serve hot. You want a crisp croque monsieur under gooey Gruyère béchamel — not a soggy sandwich or a pale unbrowned cheese blanket.