Add the vegetables, stock, tomatoes and salt, bring to a boil, reduce the heat to moderately low, and cover. Simmer just until the vegetables are tender, 10 - 12 minutes.
While the vegetables cook, whisk the sour cream and flour together in a small bowl. Add a ladleful of the broth to the mixture and whisk again. When the vegetables are tender, stir the sour cream mixture into the soup. Simmer, stirring often, for 8 - 10 minutes. Taste the soup before serving, adding a bit more salt, if necessary. We added cracked pepper at the table. Serves 5 to 6.