Preheat oven to 400°F. Combine first 6 ingredients in a large, oven-safe, soup pot. Drizzle oil over; toss to coat. Sprinkle with salt. Roast vegetables until tender and golden, stirring occasionally, about 40 minutes. Remove from oven.
Add 2 cups water to the soup pot; stir vegetables, scraping up any browned bits from bottom. Add remaining 6 cups water and all remaining ingredients. Bring to boil. Reduce heat, cover and simmer 30 minutes. Strain broth through a sieve.
Use immediately, refrigerate for several days, or freeze.
Recipe adapted from Roasted Vegetable Broth, Bon Appétit, October 1998.