Tip, in case useful:
All roux is based on starch cooked without water, which converts it to dextrin. Professionally, basic roux is sometimes made in bulk by _baking_ a fat-starch combination, but another commercial method, which I've found very useful at home, is simply to roast starch, dry (I've used both corn and potato) to the light tan color it acquires, let it cool, then soon (before it starts absorbing moisture from the air) transfer to a sealed jar. It lasts for years, unrefrigerated, if tightly closed, and you can add your favorite fat (duck, butter, etc.) before or even after combining it with a sauce. Half a pound of cornstarch spread out in a metal cake or pie pan and baked an hour or so in a (preheated!!) 400 deg F oven, occasionally stirring or tossing it, works.
A particular advantage of this is the not-always-publicized fact that some fats (duck and some others) decompose and smoke at temperatures below the 375 F or so needed for efficient dextrination. With the dextrin made in advance, you're free to use absolutely any fat you want. You can then cook it further for a dark (basically burnt) Louisiana roux, or cook the fat separately if you just want to make it smoke, or not.
(Sniffing a jar of finished dextrin reveals a familiar, slightly sweet smell that some people may have trouble placing. It's the smell of gummed envelope flaps, labels, and sheets of traditional postage stamps. All based on dextrin, cheapest and most common of the vegetable gums.)