Transfer the juices to a saucepan and simmer for 5 minutes. The sauce turned out thick enough for me, but if you want the sauce thicker, combine cornstarch with 1/4 cup water, stirring until the mixture is smooth. Pour into the simmering juices and cook, stirring, until thickened, about 1 minute. Serve the beef and vegetables with the thickened sauce. If the sauce comes out to fatty, you could put it in the fridge overnight and skim off the fat before serving.
Be sure to check out this recipe at Trade you Tuesdays!