Understand what a roux does
A roux cooks flour in fat before liquid is added. Equal weights of flour and fat are a common starting point, but the needed amount depends on the final sauce. Pale roux suits milk sauces; darker roux contributes more toasted flavor and has less thickening power.
Build a lump-free base
Melt the fat, stir in flour and cook while stirring until the raw flour aroma fades, keeping it pale for a white sauce. Whisk in liquid in small additions at first, fully smoothing each addition before adding more. Scrape corners where paste can hide.
Use a slurry when appropriate
For a glossy stir-fry sauce, mix cornstarch with a little cold liquid before adding it to hot liquid. Stir again immediately before using because starch settles. Add gradually, bring the sauce to a simmer and assess before adding more. Flour and cornstarch are not interchangeable spoon for spoon.
Troubleshoot before serving
Thin a thick sauce with its base liquid. Let a thin sauce cook long enough for its thickener to work before adding more. Strain stubborn lumps. A burned roux needs restarting; scraping blackened bits into the sauce will spread bitterness. Judge consistency at serving temperature because many sauces thicken as they cool.
Your next kitchen exercise
Whisk a small béchamel from a dedicated recipe, then divide it: thin one portion with milk and leave another thicker. Compare how each coats a spoon.
