Step 1Heat a medium, heavy-bottom pot over medium heat until hot. Add the oil, then add the carrots, onions, bell pepper, thyme and barley. Cook, stirring occasionally, until the vegetables are golden-brown, about 18 minutes, taking care that the barley does not burn.
Step 2Stir in the vegetable broth and marinara. Cover and bring to a boil over high heat, then reduce the heat to a gentle simmer.
Step 3Cook until the barley is al dente, 6 to 8 minutes. Stir in the mushrooms and zucchini, and season to taste with salt and pepper (the amount of seasoning needed will vary depending on the vegetable broth and marinara used). Cover and continue to simmer until the mushrooms and zucchini are just tender, 4 to 6 minutes.
Step 4Remove from heat, thin if desired, and season again to taste, and serve. This makes about 2 1/2 quarts of soup.