From the 2017 Thanksgiving GuideSee More
How can I keep lumps from forming in my gravy?
When you make gravy, what you’re doing, essentially, is dissolving tasty pan drippings in liquid and thickening the mixture with flour (or another starch). And how you add the flour makes all the difference. If you add the flour directly to the simmering liquid, you’ll get lumps. Such lumps form because hot liquid causes the starch molecules on the surface of the flour to almost instantly gelatinize—that is, the starches swell, burst, and become sticky. The gelatinized starch forms a waterproof coating around the lump. Squeeze open one of these lumps, and you’ll find dry flour inside.
To prevent lumps, you need to keep the grains of flour from touching one another. You can accomplish this by mixing the flour into cool liquid and then stirring this slurry into the warm or hot liquid that you want to thicken. Or if there’s fat in the pan with the drippings, you can cook equal parts flour and fat together to make a roux. In a roux, fat coats the starch granules so that they won’t stick together when they encounter hot liquid.