Adding a quart of turkey broth to a hot roux can be a recipe for lumpy gravy–but if you whisk it in gradually, you can avoid the lumps and make your gravy perfectly smooth.
Start out by making your roux right in the roasting pan, straddled across two burners; this way, you can incorporate all the flavor from those browned bits at the bottom of the pan. How much roux to make? Use 1 Tbs. of fat and 1-1/2 Tbs. flour for each cup of broth you want to add.
Once the flour is completely incorporated into the fat, cook for 2 minutes over medium heat, then slowly pour about 1/2 cup of broth into the pan and whisk constantly while you’re doing this to disperse the flour. Almost immediately, the broth thickens and looks kind of gluey. As soon as that happens, add another 1/2 cup of broth, and keep on whisking.
Keep doing this, adding the broth 1/2 cup at a time, until the gravy starts looking more like a smooth sauce than glue. At this point, it’s safe to whisk in the remaining broth and bring the gravy to a simmer to thicken.