If your gravy is too thin, first remove the meat and vegetables from the slow cooker and transfer to a serving platter. Mix 1 tablespoon cornstarch with 2 tablespoons cold water to make a cornstarch slurry. Then mix it into the beef broth and continue to cook on high for 5-10 minutes until the broth thickens and reaches desired consistency. Keep in mind that it continues to thicken as it sits. If the gravy becomes too thick, simply add a little extra broth to thin it out.