Combine tomatoes, onion, sugar, parsley, salt, pepper, and Worcestershire sauce in a medium Dutch oven. Cover and cook over medium heat 20 minutes or until vegetables are tender.
Process tomato mixture through a food mill. Return tomato mixture to Dutch oven; discard pulp.
Combine cornstarch and water, stirring until smooth; add to tomato mixture, stirring well. Bring to a boil; reduce heat, and simmer until soup is thickened. Serve soup hot or chilled; garnish each serving with 1 teaspoon plain yogurt, if desired.