Heat the olive oil in a large saucepan or dutch oven.Cook the carrot, onion and celery until tender.Add the tomatoes and broth, and bring to a boil.Add the vegetables, season to taste with salt/pepper/red pepper flakes.Return to a boil, then reduce to simmer (about 15 minutes).Either puree (if you like that consistency), or leave chunky.Ladle into bowls, then add a tablespoon of sour cream, and some green onions to each, and serve