Good to know . General rules in Roux

A roux is made out of melted fat and flour combined together.
You can create a white  roux, blonde or brown roux. All are done different by the browning of the flour first and type of sauce gravy needs to be added.
Roux can be added to liquids or liquids added to roux. Never add roux to icy cold liquids because the fat solidify to fast. If the liquid is to hot will create gelatine. Best roux is at room temperature. Also decide by practice what works for you.
Add cold Roux to a warm  liquid  or warm Roux to cool liquid is up to your choice.
Most used Roux is made out of half melted fat and half flour.
Roux needs to fully cook until taste of raw flour isn’t present.
Tip: practice and avoid lumps.  Strain if necessary , reduce to increase the thickness.