In a large saucepan, heat oil over medium heat. Add mushrooms, carrot and onion; cook and stir 5-6 minutes or until tender. Add water and barley. Bring to a boil. Reduce heat; simmer, uncovered, 12-15 minutes or until barley is tender., In a small bowl, mix flour, milk, salt and pepper until smooth; stir into soup. Return to a boil, stirring constantly; cook and stir 1-2 minutes or until thickened.