For many home cooks, just the sound of the word Roux can be enough to cause them to turn to the next page in the recipe book or close the window in their browser. However, as my idol Alton Brown says, a roux is nothing more than a chef’s glue! in other words, the sole purpose of a roux in a recipe is to bind together two core ingredients in order to create a smooth finished product. And, while the sound of the word Roux may be intimidating, a roux is really nothing more than the combination of fat and flour, normally in a 1:1 relationship (i.e., equal parts fat and flour). And notice that the type of fat is not specified. That is really left up to you.
With that said however, traditionally, butter is used as the fat to make the roux and, as you may have guessed, we get the term Roux from the French where it is used as the thickening agent for one of French cuisine’s most classical sauces: bechamel. In today’s kitchens, a roux is often used as a thickening agent for numerous dishes and accompaniments, including everything from gravy to soup, and sauce to stew.
A roux can be used to do more than just act as a binder or a thickening agent in a sauce, soup, or stew. In fact, one of the beauties of using butter or oil as the fat in the roux, is that you can brown the roux to whatever level that you like before adding the additional liquid ingredients. In other words, if you just simply mix equal parts butter or oil with flour and allow it to simply come together before adding your other liquid ingredients, you will get a very light sauce with little to no flavor from the roux itself. On the other hand, if you let the roux brown quite a bit, almost to the color of mahogany, you can bring out unique subtle flavors from the browning of the fat that act to enrich your sauce. So, if you want your sauce to be very bold and rich, take the roux to the next level by letting it brown quite a bit before adding your liquid ingredients. On the other hand, for many applications, like macaroni and cheese or a béchamel sauce in general, just a very light color is all that is needed since the cheese will give you the flavors that you want and not the roux.
I like to think that a good rule of thumb for most sauces and soups is to think of the number 3: 3 tbsp of butter (or oil or bacon fat!) with 3 tbsp of flour. If you use butter, be sure to let the butter melt before adding the flour. Using a whisk to combine the fat and butter for the roux works best. And then, just keep whisking every 15 to 30 seconds until the roux comes the color of your liking. If you want a thinner sauce, go with 2 tbsp of each and if you want a much thicker sauce go with 4 tbsp. And, lastly, if you really want to kick up your roux, render some bacon or pancetta and use the fat for the roux . . . it works great for macaroni and cheese.