How to Make It
Set a large colander or a fine-mesh strainer over a large bowl. Scrape vegetables and drippings from reserved baking sheets into colander. Discard vegetables. Pour ½ cup broth onto each baking sheet, carefully scraping up any browned bits; add to drippings in bowl.
Pour drippings into a fat separator or a tall, narrow container. Let stand for 10 minutes. Skim off fat and discard. Transfer remaining drippings to a medium saucepan. Add remaining 2 cups broth and bring to a boil over medium-low.
Meanwhile, whisk cornstarch and 3 tablespoons cold water in a small bowl until smooth. Slowly whisk cornstarch mixture into simmering broth and cook until gravy coats the back of a spoon, about 1 minute. Whisk in butter and vinegar; season with salt and pepper.
Remove from heat and cover to keep warm until ready to serve.
Use cornstarch instead of flour to thicken the gravy without the extra step of making a roux.