Classic Croque Monsieurs Recipe

Make a roux

In a medium saucepan over medium heat, melt 4 tablespoons unsalted butter. Add 4 tablespoons all-purpose flour and whisk constantly for 1–2 minutes until the mixture is bubbly and lightly golden but not browned.

Add milk

Gradually whisk in 2 cups whole milk and 1/2 cup heavy cream, pouring slowly to avoid lumps. Continue whisking until the sauce comes to a simmer and thickens, about 4–6 minutes.

Season and finish

Reduce heat to low. Whisk in 1 teaspoon Dijon mustard, 1/4 teaspoon grated nutmeg, 1/2 teaspoon salt and 1/4 teaspoon black pepper. Remove from heat and stir in 1 cup grated Gruyère and 1/4 cup grated Parmesan until melted and smooth. Taste and adjust seasoning. Keep warm; if it thickens too much, whisk in a tablespoon of milk to loosen.

Prepare bread and butter

Lightly butter one side of each slice of the 24 bread slices using the 6 tablespoons softened butter; these will be the outside surfaces for grilling.

Assemble

Place 12 unbuttered bread slices on a work surface, buttered side down. Spread a thin layer (about 1 tablespoon) of the warm Mornay sauce on each of these slices. Top each with 2 slices of ham and about 2 tablespoons (scant) of grated Gruyère (from the 4 cups set aside). Place the remaining 12 bread slices on top, buttered side up, to form 12 sandwiches.

Option for smaller passed portions

If making passed hors d'oeuvre portions, cut each sandwich into quarters (12 sandwiches = 48 bite-sized pieces). Alternatively, cut sandwiches into triangles or use 2-inch cookie rounds before grilling.

Pan-grill (preferred for even browning)

Heat a large skillet or griddle over medium heat. Working in batches, place sandwiches buttered-side down in the skillet. Cook 3–4 minutes until golden brown, press gently with a spatula or press, then flip and cook 2–3 minutes until the second side is golden and cheese begins to melt. Transfer sandwiches to a baking sheet, seam-side down.

Top with sauce and cheese

Spoon a generous tablespoon of Mornay sauce onto the top of each sandwich (or over each quartered piece for passed portions). Sprinkle additional Gruyère and a little Parmesan over the sauce, about 1 tablespoon grated cheese per sandwich (adjust for smaller portions).

Finish under broiler

Preheat broiler to high. Place the baking sheet with sandwiches 4–6 inches from the heat and broil 2–4 minutes until topping is bubbly and golden brown — watch carefully to avoid burning. Remove and let rest 1–2 minutes.

If made whole, slice sandwiches in half or quarters. For passed hors d'oeuvres, arrange bite-sized pieces on platters and garnish lightly with 2 tablespoons chopped parsley if desired. Serve warm.