Cook the sauce on a low-medium heat, while stirring occasionally for 15-20 minutes, or until the sauce seems set.
Whisk in the flour and reduce to sauce consistency, skimming occasionally and adding more stock as necessary.
Continue to cook until the sauce has reduced by three quarters.
mid-14c., from Old French sauce, sausse, from Latin salsa "things salted, salt food," noun use of fem. singular or neuter plural of adjective salsus "salted," from past participle of Old Latin sallere "to salt," from sal (genitive salis) "salt" (see salt (n.)).
Meaning "something which adds piquancy to words or actions" is recorded from c.1500; sense of "impertinence" first recorded 1835 (see saucy, and cf. sass). Slang meaning "liquor" first attested 1940.
mid-15c., "to season," from sauce (n.). From 1862 as "to speak impertinently." Related: Sauced; saucing.