Croque Monsieur with 10-Minute Mornay

By Sprig Originals ✓ · cooked up in Sprig

The Paris café classic, built on a mornay sauce that comes together in the time it takes to toast the bread. Ham and Gruyère go between buttered slices, the whole thing gets blanketed in sauce, then a hard blast under the broiler blisters the cheese into dark, bubbling freckles instead of just melting it flat. That contrast — molten inside, scorched and crackling on top — is the whole point.

Ingredients

• 3 tbsp unsalted butter
• 3 tbsp all-purpose flour
• 1 1/2 cups whole milk, warmed
• 1 tsp Dijon mustard
• 1/8 tsp freshly grated nutmeg
• 1/2 cup Gruyère, finely grated
• 1/2 tsp kosher salt
• freshly ground white or black pepper

• 8 slices sturdy white sandwich bread or pain de mie, 1/2-inch thick
• 3 tbsp unsalted butter, softened
• 2 tsp Dijon mustard
• 8 oz thinly sliced good-quality ham (Paris ham or Black Forest)
• 2 cups Gruyère, coarsely grated (about 6 oz)
• 1/4 cup Parmigiano-Reggiano, finely grated
• snipped chives, for serving (optional)

Instructions

1. Heat the oven to 400°F and set a rack in the upper third, about 6 inches below the broiler element. Line a rimmed baking sheet with parchment. Warm the milk for the mornay in the microwave or a small saucepan until steaming but not boiling — warm milk is what lets the sauce come together in minutes without lumps.
2. Make the mornay: melt 3 tablespoons butter in a small saucepan over medium heat. Whisk in the flour and cook, whisking constantly, for 90 seconds — the paste should smell faintly nutty and look pale gold, not browned. Pour in about 1/3 of the warm milk while whisking hard; it will seize into a thick paste. Whisk until smooth, then add the rest of the milk in two more additions, whisking smooth each time.
3. Bring the sauce to a bare simmer and cook 3 to 4 minutes, whisking, until it thickly coats the back of a spoon and no longer tastes floury. Off the heat, whisk in 1 teaspoon Dijon, the nutmeg, 1/2 teaspoon salt, several grinds of pepper, and the 1/2 cup finely grated Gruyère until glossy. Total elapsed time should be about 10 minutes. Press a piece of plastic wrap directly onto the surface if it will sit more than a few minutes.
4. Lay the 8 bread slices on the baking sheet and toast in the 400°F oven for 4 minutes, flipping once, just to dry them out and firm the crumb. This is the insurance policy against a soggy sandwich. Leave the oven on.
5. Butter one side of each toasted slice with the softened butter, then spread a thin film of Dijon over 4 of them. Divide the ham among those 4 slices, folding and ruffling it so it sits tall rather than flat, then add about 1 cup of the coarsely grated Gruyère total, split among the four. Close each sandwich with a remaining slice, buttered side down.
6. Spoon 1 to 2 tablespoons of mornay over the top of each sandwich and spread it to the very edges — the corners scorch beautifully and the sauce protects the crust. Divide the remaining mornay over the tops in a thick, uneven layer, then scatter on the remaining 1 cup Gruyère and all the Parmigiano. Leave the cheese loose and peaked; those high spots are what blister.
7. Bake at 400°F for 10 to 12 minutes, until the sauce is bubbling around the edges and the cheese has fully melted into a molten sheet.
8. Switch the oven to broil on high and move the sheet to that upper rack. Broil 2 to 4 minutes, rotating the pan once and never walking away, until the surface is spotted with deep amber and near-black freckles and the whole top is puffed and blistered. You want visible dark spots, not a uniform tan — pull it the moment the spots start joining together.
9. Rest 3 minutes so the interior cheese sets enough to slice cleanly. Cut each sandwich in half on the diagonal, shower with chives, and serve immediately with a sharply dressed green salad.

Chef's tips

Nutrition (per serving, estimated)

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