Roux
Equal parts butter and flour, cooked briefly, thicken a sauce.
What to do
- Melt the butter and add the same amount of flour.
- Stir a minute or two so the flour loses its raw taste.
- Only then pour in cooler liquid, in a stream, and stir out the lumps.
A common slip. Roiling stock into a very hot roux, all at once, and the sauce lumps before it comes together.