Strain the pan drippings into a fat separator or large liquid measuring cup; set aside. Place the empty roasting pan across two burners over medium-high heat. Add a splash of water and scrape up any browned bits; add them to the drippings.
Spoon or pour off 1/2 cup of the fat from the drippings and transfer back into the roasting pan. (If you don't have enough fat to make 1/2 cup, add vegetable oil.) Set the roasting pan over medium-high heat. Whisk in the flour and cook, whisking, until smooth and golden, 2 to 3 minutes.
Whisk in the defatted drippings and enough broth to make 8 cups total liquid. Bring to a simmer and cook, whisking, until thickened, 15 to 25 minutes. Season with salt and pepper.