Heat pan drippings in a medium saucepan over medium. Add onions, and cook, stirring occasionally, until tender, about 5 minutes. Sprinkle flour over onions, stirring constantly; cook, stirring constantly, until flour is golden brown, about 2 minutes.
Gradually whisk in 4 cups of the broth. Cook, stirring often, just until mixture comes to a boil and is smooth and thick, about 5 minutes. If mixture is too thick, add up to 1 cup broth, 1/4 cup at a time, until mixture reaches desired consistency. Stir in sherry vinegar, salt, and pepper. Serve with roast and vegetables.