In 10-inch skillet, heat butter over medium heat until hot. Cook mushrooms and onion in butter, stirring occasionally, about 4 minutes or until mushrooms are tender. Add green beans, bell pepper and salt. Cook 6 to 7 minutes, stirring occasionally, until beans are tender.
Mix chicken broth and flour with whisk or fork; add to skillet. Cook and stir 3 to 4 minutes or until thickened. Remove from heat; stir in yogurt. Sprinkle with walnuts.