Ingredients
Method
Make the Béchamel
- Melt the butter in a saucepan over medium heat. Add the flour and whisk constantly for 1–2 minutes until it forms a smooth pale roux. Do not brown the roux. Croque monsieur béchamel should stay creamy and pale, not taste like toasted flour. Slowly whisk in the warm milk in several additions, making sure each addition is smooth before adding more. Cook for 4–6 minutes, whisking often, until thick, glossy, and spoonable. Season with salt, black pepper, and nutmeg. Stir in 80g grated Gruyère until melted. The béchamel should be thick enough to sit on top of the sandwich without running straight off. If it is thin and loose, keep cooking. Runny béchamel ruins the whole structure. Using warm rather than cold milk, and adding it gradually in stages, is what keeps the roux from seizing or forming lumps — each addition needs to fully smooth out before the next goes in, giving the starch time to properly hydrate and thicken evenly.
Prepare the Bread
- Spread a thin layer of softened butter on one side of each slice of bread. This buttered side will face the pan. Spread Dijon mustard lightly on the inside of 4 slices. Do not overdo it. Mustard should sharpen the sandwich, not dominate it. A restrained amount of mustard is meant to provide a background sharpness that cuts through the richness of the béchamel and cheese, rather than becoming its own dominant flavour note competing with everything else.
Assemble the Sandwiches
- Place 4 slices of bread butter-side down. Add a layer of Gruyère, then the ham, then another light layer of Gruyère. Close with the remaining bread slices, butter-side facing out. Keep the ham folded neatly so it reaches the edges without hanging out too much. Messy overhang burns under the broiler. Cheese placed directly against both the bottom bread and again above the ham means the melting cheese binds the whole interior together from both directions once griddled and broiled, rather than leaving the ham as a loose, separate layer.
Griddle the Sandwiches
- Heat a large skillet or griddle over medium-low to medium heat. Cook the sandwiches for 3–4 minutes on the first side until deeply golden and crisp. Flip and cook another 3–4 minutes until the second side is golden and the cheese inside starts melting. Do not use high heat. The bread will brown before the cheese melts, and then you are stuck with a pretty exterior and a cold centre. This griddling stage is doing real, separate work from the broiler that follows — it's specifically what crisps the bread's exterior and begins melting the interior cheese, tasks the broiler alone (focused entirely on the top) wouldn't accomplish for the sandwich's base and sides.
Top with Béchamel and Cheese
- Transfer the griddled sandwiches to a baking tray. Spoon a generous layer of béchamel over the top of each sandwich, spreading it close to the edges. Scatter the remaining Gruyère over the béchamel. Finish with a little black pepper and a tiny pinch of nutmeg if using. Spreading the béchamel close to the edges, rather than leaving a wide bare border, ensures the whole top surface bubbles and browns evenly under the broiler rather than leaving dry, unsauced patches of bread exposed to direct heat.
Broil Until Golden
- Place under a hot broiler for 3–5 minutes, until the top is bubbling, blistered, and deeply golden-brown in spots. Watch constantly. This goes from perfect to burnt fast. Walking away here is how you destroy it. The béchamel and cheese topping is thin enough, and broiler heat intense enough, that the gap between "just right" and "scorched" is genuinely only a matter of seconds — this is one step in the whole recipe that tolerates no distraction.
Serve
- Let the sandwiches rest for 1–2 minutes, then serve hot while the top is bubbling, the cheese is melted, and the bread is still crisp underneath. This brief rest lets the molten béchamel and cheese firm just slightly so the sandwich holds together when cut or picked up, while everything is still fully melted and bubbling hot.
Notes
The two-stage heat approach — griddling first, then broiling — is worth understanding as the single most important structural decision in this recipe rather than an extra step for convenience. Griddling alone would leave the top bare and unfinished; broiling alone, without first griddling, would leave soft, pale, unbrowned bread beneath a nicely bubbled top. Only the combination produces a sandwich that's properly crisp on the outside and bottom, fully melted throughout, and bubbling and blistered on top — each stage doing a job the other genuinely can't accomplish on its own.
The béchamel's thickness is worth monitoring carefully rather than trusting the clock alone, since flour's thickening behaviour can vary slightly depending on how it was measured and how vigorously it was whisked. The visual and textural cue — coating the back of a spoon, holding its shape rather than running — is the more reliable indicator than a fixed cooking time, and it's worth continuing to cook past the stated minimum if the sauce still looks loose.
Restraint with the Dijon mustard reflects its intended role in this sandwich rather than an arbitrary limit. A generous, thick layer of mustard would compete directly with the béchamel and Gruyère for dominance, throwing off the sandwich's balance. Used sparingly, mustard instead functions as a background sharpness that lifts and cuts through the richer components without ever becoming the most noticeable flavour.
The final broiling stage's narrow margin for error is a genuine characteristic of the technique, not an exaggeration. A thin layer of béchamel and cheese under intense, direct broiler heat moves from unmelted to perfectly bubbled and browned to burnt within a very short window — staying at the oven and watching continuously, rather than setting a timer and walking away, is the only reliable way to catch the sandwich at its ideal point.