Puree tomatoes in a food processor or food mill, then press through a sieve to remove seeds. In a large mixing bowl, beat the pureed tomatoes, tomato paste, buttermilk, and oil.
Toss pureed avocado with 1 tablespoon lemon juice to hold the color. Add the avocado, remaining lemon juice, and parsley to the tomato mixture; stir to mix well. Season to taste with salt and pepper, and a generous number of drops of hot pepper sauce.
Refrigerate several hours before serving.
At serving time, taste soup for seasonings. Ladle into individual bowl and have guest garnish their portions with cucumber and sour cream. Pass hot pepper sauce around to add more piquancy.
Makes 8 to 10 servings.