Preparation Heat oil in heavy large pot over medium heat. Add leeks and sauté until tender, about 6 minutes. Add tomatoes and their juices, broth, chopped dill and cayenne and bring to boil. Reduce heat and simmer uncovered until tomatoes are very soft and flavors blend, about 20 minutes. Working in batches, puree soup in processor until smooth. Return to same pot. (Can be made 1 day ahead. Cover and chill.) Bring to simmer over medium-low heat. Season with salt and pepper. Gradually whisk in sour cream (do not boil). Ladle soup into bowls. Arrange cheese atop each. Garnish with dill sprigs.
Preparation Heat oil in heavy large pot over medium heat. Add leeks and sauté until tender, about 6 minutes. Add tomatoes and their juices, broth, chopped dill and cayenne and bring to boil. Reduce heat and simmer uncovered until tomatoes are very soft and flavors blend, about 20 minutes. Working in batches, puree soup in processor until smooth. Return to same pot. (Can be made 1 day ahead. Cover and chill.) Bring to simmer over medium-low heat. Season with salt and pepper. Gradually whisk in sour cream (do not boil). Ladle soup into bowls. Arrange cheese atop each. Garnish with dill sprigs.