Using a fine-mesh sieve, strain the pan drippings into a large glass measuring cup. Skim off the fat, reserving 1/3 cup in a small bowl. Place the roasting pan over medium heat, add 2 cups broth and bring to a boil. Stir, scraping up any browned bits, until the broth reduces slightly, about 2 minutes.
Add enough remaining broth to the cup with the pan drippings to measure 1 1/2 cups, pour it into the roasting pan and bring to a boil. Whisk the flour into the reserved 1/3 cup fat, then whisk the flour mixture into the pan juices; return to a boil. Simmer, whisking, until thickened, about 5 minutes. Season.