Broil beef 4-6 in. from the heat for 6-9 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Set aside and keep warm.
Meanwhile, in a large skillet, bring broth to a boil. Add mushrooms and onion; cover and simmer until tender, about 5 minutes. In a small bowl, whisk the flour, cornstarch and water until smooth. Whisk into broth. Bring to a boil. Cook and stir for 2 minutes or until thickened and bubbly. Remove from the heat.
In another small bowl, combine the yogurt, paprika, mustard and garlic powder; stir into broth mixture until smooth. Cut beef diagonally into thin strips; add to broth mixture. Cook and stir over low heat until heated through. Serve with noodles if desired.