Béchamel

Steps

1. Infuse5 min
   small pan, below a simmer
   • milk
   • bay leaf
   • onion
   milk (+ bay leaf, + onion), warmed until it steams but does not bubble, then the aromatics lifted out. Warming is not optional even without them — it is what lets the starch disperse before it swells
2. Saute3 min
   medium-low, constant stirring
   • butter
   • flour
   butter until melted, + flour, stirred into a smooth paste and cooked until it loosens, smells like plain biscuits and stops smelling like raw flour. This step is cooking off the flour's raw taste, not browning it — colour here means less thickening power later
3. Whisk3 min
   a splash at a time for the first third
   + warm milk, whisking each addition to smooth before the next. The first additions are the only ones that can go wrong: with little liquid present the paste stays thick enough to hold lumps together, so keep it tight and glossy rather than flooding it
4. Simmer8 min
   low, lazy bubbles at the edge
   stirring and scraping the base, until the sauce coats a spoon and a finger drawn through it leaves a clean line. Thickening happens over a narrow band above about 85°C as the starch granules swell and burst; below it the sauce stays thin however long it sits, and the extra minutes also cook out the pasty flavour
5. Season1 min
   off the heat
   • nutmeg
   • black pepper
   • cheese
   + salt + nutmeg (+ pepper, + grated Gruyère for a Mornay), stirred until smooth. Cheese goes in off the heat: above about 65°C its proteins tighten and squeeze out fat, and the sauce turns grainy with an oil slick on top