Croque Monsieur

Method

1. mix3 min
   medium-low
   butter melted, + flour whisked in and cooked until it smells of biscuit but stays blond — this is the roux; the raw-flour taste burns off in these three minutes and nowhere later in the recipe
2. whisk2 min
   medium, adding in thirds
   + warm milk, poured in stages and whisked smooth before the next addition — cold milk hitting hot roux sets lumps instantly, and warm milk added all at once drops the pan below the temperature where the starch swells
3. simmer4 min
   gentle bubble
   + nutmeg + salt + pepper, stirred until the sauce coats a spoon and holds a drawn line = béchamel. Off heat, + a third of the gruyère, stirred until melted — adding cheese to a simmering sauce breaks it, since the proteins tighten and squeeze their fat out
4. toast4 min
   dry, medium heat or 400°F oven
   bread slices, until pale gold and firm but not crisp through. Dry toast builds a wall that keeps the béchamel from soaking the crumb to paste; buttered bread cannot do this because the fat has already occupied the pores
5. top3 min
   two slices spread with dijon + a spoon of béchamel + ham + half the remaining gruyère, closed with the second slices, then the whole tops blanketed with the rest of the béchamel edge to edge + the last of the gruyère. Bare bread at the rim will scorch before the center browns
6. broil4 min
   high, rack 4-5 inches from the element
   sandwiches on a foiled sheet, until the top blisters and goes brown in patches. Watch it — the sugars in the milk and the cheese fat go from bronze to bitter in about forty seconds, and the béchamel must be hot enough to bubble at the edges or the inside is still cool
7. rest2 min
   off heat
   the sandwiches, so the molten cheese sets enough to be cut rather than dragged