How would you make a roux efficiently?

In a roux, the starches are not normally gelatinized during the production of the roux--in fact, if you make your roux with oil rather than butter, there is no water present (other than latent moisture from the flour) to do so.

Roux thickened sauces must normally be brought to the boil; this is when the actual gelatinization takes place.

. . .

Roux is easy to prepare in larger batches, stove top. Simple use a larger pan, such as a six quart dutch oven.

Melt your butter or add your oil, then whisk in the flour. Cook for a couple of minutes (after the bubbling subsides if using butter), and you are good to go. Don't be afraid to turn the heat up as long as you are standing by, watching and stirring to prevent burning from hot spots.

Butter can foam up considerably, so use a large pot in comparison to the quantity of roux.

For dark roux, I would start stove top, and finish in the oven to reduce the likelihood of taking the browning too far, and reduce the labor of stirring required.

The roux can be refrigerated. You can scoop it with a spoon and add it to sauce at the simmer and whisk, and it will thicken almost instantly.

See also: Is there a technique for making larger batches of roux?

My easiest and most precise method isn't to roast the flour, but to "fry the flour" instead. This method provides much better control and ensures the roux is precisely as dark as you like.

--- Put one cup of flour in a non-stick pan.
--- Turn the heat high as if you were using the conventional method.
--- Using a non-stick spatula, continue to keep turning the flour.

Within a minute or so, it will start to smelling like popcorn. Watch it with a bright light, and you will see it turn very light brown. The flour will also start to clump a little.

--- "Fry" for about 5 minutes or so, until evenly as light brown as you'd like.
--- Turn heat off or down --- Add a stick of butter, and finish the roux without adding any more heat.

You will notice that once you add butter, it immediately becomes much darker than you thought it would. The oil in the pan to conducts heat much better, and it now will cook extremely fast, so make sure that the heat is not too high.

If I'm using the roux right then, I start adding my liquid after the butter is mixed in.

I've found that I can control the heat perfectly this way and it won't be over or under cooked. Using the standard method with my electric stove either over or under cooks it. The only way to cool it fast enough when too hot is to put it on granite, which is not the best.

Thanks for validating this method @Barry, (and helping me discover that my method was not unique ).

Questions
-- How does the gluten in my bread flour affect it?
-- If you use another oil, do you need add liquid to make sure it thickens, or just wait until you use it in a sauce later?

A trick I learned in culinary school, is to pre"toast" the flour in the oven (on a sheet pan); prior to mixing in with your fat of choice. This cooks out the raw starchy taste. As far as sous vide is concerned; I actually came upon this link in an attempt to see if / how others have had any success with roux at sub-boiling temperatures (outside of a pressure cooker; which obviously allows you to raise the boiling point). I guess I'll just have to experiment a bit to see if I can get decent results.