Heat olive oil in large heavy saucepan. Add onions and garlic; cook 5 minutes or until transparent. Add flour, whisk over low heat 3 to 5 minutes. Do not brown. Whisk in chicken broth, removing all lumps. Add 1/2 cup fresh basil. Heat mixture to a boil; reduce heat to simmer and cook 5 to 10 minutes. Whisk in  diced tomatoes. Simmer 15 minutes. With large spoon, skim froth from top as it forms. Whisk in cream; heat to a simmer. Skim again, if necessary. Add sugar and season with salt and white pepper. Ladle into warm cups or bowls and garnish with remaining basil and fresh crabmeat, if desired. 8 cups