Heat butter, mushrooms, and garlic in a medium skillet, 7-8 minutes or until mushrooms are tender, stirring occasionally. Remove from skillet and set aside.
In same skillet, cook beef 5-7 minutes, stirring frequently, until brown. Stir in 1 c. broth and salt. Bring to a boil, cover, reduce heat, and let simmer 15 minutes.
Stir remaining 1⁄2 c. broth into flour. Stir well, then stir into beef mixture. Add mushroom mixture, bring to a boil, and boil 1 minute, stirring constantly.
Stir in yogurt and sour cream. Heat just until hot. Pour over cooked noodles. Sprinkle with green onions.