Skim the fat from the chicken broth by chilling it until the fat floats to the top and can be whisked away. Combine the flour with the liquid from the tuna in a saucepan. Gradually stir in the broth and cook over low heat, stirring constantly, until it has thickened. Stir in the tomato sauce, onions, mushrooms, parsley, and tuna. Cover the pot and simmer until heated though. Stir in the yogurt and continue cooking just until hot...do NOT boil. Serves 8 Per serving: Calories--116.3 Carbohydrate--9.1 grams Fat--1.1 grams