Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Meanwhile, melt 1 tablespoon of butter in a skillet over medium heat. Add the beef, and cook until meat is no longer pink inside. Remove from skillet and keep warm.
Melt 2 tablespoons of butter in the same skillet over medium-high heat; stir in mushrooms and cook until tender. Pour in chicken broth and bring to a simmer. Once the broth is hot, stir the cheeses and salt until the cheese has melted.
Mix together the water and cornstarch and slowly add to the simmering broth. Cook for 1 minute until thick and clear, stirring continually. Serve over pasta and beef medallions.
