Heat oil in large saucepan. Add mushrooms, onion, bell pepper, and garlic. Cook, stirring, for 8-10 minutes over medium heat. Add water or broth, barley, wild rice, mustard and seasonings. Bring to a simmer. Cook uncovered for 50 minutes to 1 hour over medium-low heat; stirring occasionally.
Remove soup from heat and let stand about 10 minutes before serving. Ladle into bowls and serve if desired with warm dark bread. Makes 6-8 servings.