I thought I'd take a moment to talk about roux (pronounced "roo"). It's one of the most basics of basics in western cooking, and in particular in recipes with French origins and/or influence. It's mostly used for adding thickness and richness to soups and sauces, but it can also be used for flavor.
In a nutshell, roux consists of equal weights of flour and clarified fat. Classically, the fat would be clarified butter, but it doesn't always have to be so. Before we get into hacking roux, let's talk about how to make it.
If you had access to a scale, I might have you weight out a few ounces (or grams) of butter, and then an equal weight of butter. But you probably don't, and that's okay. Scales are for bakers (such as myself), not necessarily for cooks. You can just start with a couple of tablespoons of butter. Clarified would be great, but if you only have whole butter, that's fine too. My one recommendation is that you at least use unsalted.
Melt the butter in a pan over medium heat and then sprinkle in a tablespoon or two of flour. Cake flour would be dandy, but all-purpose flour is fine too. Start with a smaller amount for now. You can always add more flour, but you can't take it away. Whisk it together until it forms sort of a paste. If it's thing enough that you could pour it, then it's too thin; add more flour. If it's thick enough that all of the flour won't mix in, then you've added too much. There are certainly uses for "thin roux" and "thick roux", but we won't cover them here.
Once you have the proper consistency, all that's left is to cook it to the right level of doneness. At the very least, you'll want to cook out that raw flour taste. Make sure to whisk occassionally as it cooks, so that it doesn't cook unevenly or burn on the bottom. If cooking out the raw cereal flavor is the furthest you go, it's called a white roux, or "roux blanc". If you take it to a slightly more tan color, it's called a blonde roux ("roux blond"). You can take it even further to a brown color that almost looks like milk chocolate ("roux brun").
These three are the most common types of roux. As the roux gets darker, the flavor increases, but the thickening power also decreases. There is another type of roux, typically seen in cajun cooking, called a black roux, or "roux noisette". It's not actually black, but it's close, with kind of a dark red tint to it. If it were completely black, it would be burned, and would add nothing in the way of flavor except for bitterness. But a proper roux noisette can add a deep flavor to cajun stews and gumbos that can't be found anywhere else. Unfortunately, it adds next to no thickening, but that's okay. Gumbo uses other ingredients for thickening.
Now you know about the classic roux. What's it good for? Mostly soups and sauces. And in fact, in most cases it will be used as the basis of a mother sauce, which is also mostly used for soups and sauces. Most home cooks know how to strain the fat from a Thanksgiving turkey, add a little flour and turkey juices, and turn it into a gravy. But obviously, turkey fat is not the same thing as butter fat, and that's what leads us into hacking roux.
You already know that a classic roux consists of equal weights of flour and clarified fat. Why then did I tell you that it was okay to use whole butter? Those of you who read my post on dairy percentages know that whole butter is not 100% fat. It contains anywhere from 15 - 20% water, plus a small amount of milk protein, which are not ingredients in a classic roux. Here's the thing: with such a small amount of roux as we would be making at home, most of the water is going to evaporate before it becomes an issue. And unless we're making a roux brun or roux noisette, we probably don't need to worry all that much about the milk solids burning.
So now you know that it's okay if sometimes other things are present when making a roux. In fact, it's very common for French chefs to sweat a few veggies in butter and then add flour and form a roux. When ingredients other than the fat and flour are present when you make a roux, it's known as a "compound roux". Another excellent example of a compound roux might be when somebody cooks up a bunch of breakfast sausage, removes most of the meat from the pan, adds a little flour and makes up a delight called country or sawmill gravy.
The fact that you can use turkey fat or sausage fat tells us that you don't need butter. Any fat will work for a roux, though I must say that I've never found much of a use for a roux that consists primarily of unsaturated fats. Duck fat, bacon fat, beef fat, it's all fair game for roux. Whichever fat you use primarily depends upon what your final intended dish is.
I should make a note while I'm at it. It's not uncommon for restaurants to make up several pounds of roux at a time and keep it in the cooler until it's needed. Chances are it's going to be used in the next week anyway. In this case, they will generally use clarified butter, and make up separate batches of blanc, blond and brun roux, or whichever type they know they'll need on a regular basis. That will save them a little time during dinner rush, when they don't necessarily have time to make up a roux for each and every plate that goes out to customers. Home cooks will rarely, if ever, need to do such a thing. But I thought I'd mention it.