In a 3- to 4-quart pan over medium heat, melt butter. Add flour and stir for about 30 seconds. Whisk in tomato juice and cream until smooth.
Add canned tomatoes (including juice), roasted peppers, garlic, sugar, tarragon, and cayenne. Stir often until soup just begins to simmer, about 10 minutes. Add salt and pepper to taste.
Ladle into bowls. Garnish each with a spoonful of sour cream, chopped fresh tomato, and parsley.