Bring to a boil over medium-high heat, whisking occasionally. Reduce heat to medium, and gently boil, whisking occasionally, 45 minutes or until thick enough to coat the back of a spoon. Season with salt to taste.
Test Kitchen Tip: To make ahead: Cool gravy 45 minutes. Cover and chill up to 3 days. Add a few tablespoons of broth, and reheat over medium heat.