In wide heavy saucepan over medium-low heat, warm olive oil. Add shallots and cook, stirring often, until softened but not browned, about 5 minutes. Add tomatoes, thyme, tomato juice, and seasoning. Bring to boil, reduce heat to low and simmer, stirring occasionally for 30 minutes.
Pass soup through food mill, or process with stick blender. If taste and texture are a little thin, return soup to very low heat and cook, stirring frequently, until thick and flavorful. Remove from heat and cool to room temperature. Cover and refrigerate until well chilled.
Salsa: Peel, pit and dice avocado. In medium bowl, toss avocado with shallot, lime juice, cilantro and olive oil. Season with salt. Ladle soup into serving bowls. Top each with a spoonful of salsa and a spoonful of sour cream.
(Adapted from Williams-Sonoma TASTE Magazine, Red, Ripe and True by Deborah Madison (Summer 2002).