Once the roast has finished cooking, remove the meat from the sauce. Place the sauce in a saucepan. In a small bowl, stir 1-2 tablespoons of cornstarch, depending on desired thickness, together with 2-4 tablespoons of beef broth. Stir the cornstarch mixture into the sauce. Simmer, stirring regularly, until it reaches the desired thickness. The gravy is great over the meat, over rice, or on mashed potatoes.