Sauté onion in hot oil in a Dutch oven over medium-high heat 5 to 6 minutes or until lightly browned. Add carrots and mushrooms, and cook, stirring occasionally, 8 to 10 minutes or until mushrooms are tender. Add remaining ingredients, and bring to a boil. Reduce heat to medium, and simmer, stirring occasionally, 5 to 6 minutes or until orzo is al dente and soup is thoroughly heated.