Place the avocado, reserving a quarter of 1 to chop and garnish the top, in a food processor. (Brush reserved avocado with lime juice to prevent oxidation and wrap until needed.) Add 1 cup of the vegetable stock and the sour cream. Pulse until smooth. Transfer to a bowl. Stir in remaining stock, lime juice, Tabasco and salt. Chill at least two hours. Pour into chilled soup bowls, garnish with chopped avocado and tomato, and serve.