Croque Monsieur

Variations

• Croque madame: Top with a runny fried egg.
• Mushroom version: Add sautéed mushrooms.
• Chicken croque: Use chicken instead of ham.
• Mini croques: Make small ones on baguette as a snack.

Tips

• Whisk warm milk in gradually for a smooth, lump-free béchamel.
• Gruyère gives the signature nutty flavor, but mozzarella or cheddar work.
• Pile cheese on top and brown it under the broiler for the best crust.
• One runny egg turns it into a croque madame — break the yolk and dig in.
• There are two separate browning events and they cannot be combined. The sandwich is toasted in a pan first and only then gets its béchamel and its second heat, and the reason is in browning: while water is present a surface stalls at 100°C and essentially nothing browns. A slice of bread under a wet sauce never gets past that, so it steams into a soft pad. Build the whole thing raw and bake it once and you get browned cheese sitting on limp bread — which is the commonest way this goes wrong.
• The béchamel is what lets the cheese brown instead of splitting. Dairy and heat gives the fix for a clumping cheese sauce as lower heat plus a little starch, "which gets between the casein and stops it knitting" — and a béchamel is exactly that, flour and milk. Cheese alone under a 250°C grill separates and weeps oil; cheese on a bed of béchamel has the starch working underneath it the whole time. So the sauce is structural rather than a richness, which is what separates this from cheese on toast.
• Why Gruyère, and what the substitutes actually do. The same guide sorts cheeses by how they melt, and Gruyère is a cooked-curd alpine cheese that melts smooth and browns well. The tip above offers cheddar or mozzarella and it is worth knowing what changes: cheddar is the guide's "mid-aged" case and goes oily and clumpy if the heat is high, and mozzarella stretches but browns poorly and gives off water. If you use either, keep the grill lower and give it longer.
• Where it sits in the family. Francesinha is this sandwich with the sauce and the meat multiplied wholesale, grilled cheese keeps only the cheese, and jambon-beurre is the same ham and bread with no heat at all.