kwplee

There are two names for 白汁 in English, actually in French - Béchamel and Velouté.

Béchamel - white sauce based on milk, a white roux and studded onion (using bay leaf 香葉 and clove 丁香 pressed into an onion).
Velouté - literally soft, velvety, smooth.  A basic white sauce made from stock and blond roux.
Roux - cooked mixture of flour and butter in the ratio 1:1.  There are 3 stages of roux (means colour)- white, blond and brown.