In a Dutch oven, heat oil over medium heat. Add carrots, celery and onion. Cook and stir until crisp-tender, about 10 minutes. Add cauliflower, mushrooms, salt and pepper; cook and stir for 5 minutes. Stir in broth, rosemary and sage. Bring to a boil; reduce heat. Simmer, covered, until vegetables are just tender, 10-12 minutes.