Cut the beef into 1 inch strips. Salt and pepper the strips. Heat the butter in Dutch oven and brown the beef, turning frequently to brown evenly. Set the meat aside and cook the onions until they are transparent. Set the onions aside. Add the flour to the drippings and mix well. Return the meat and onions to the Dutch oven, add the beef base (which has been combined with the beef broth) and the can of consomme. Stir the mixture until it is smooth. Add the mustard, mushrooms and sherry. Cover and cook very slowly for 1/2 hour. Add the sour cream and serve over the pasta.
Cook the pasta according to package directions (6 to 7 minutes in boiling salted water). Drain and then toss with 1 tablespoon of butter to keep noodles from sticking.