Puree tomatoes, then press through a sieve to remove seeds. In a large bowl, beat the pureed tomatoes, tomato paste, buttermilk and oil. Toss pureed avocado with 1 TBS lemon juice (to hold color). Add avocado, remaining lemon juice, parsley to the tomato mixture; stir to mix well. Season to taste with salt and pepper, and hot pepper sauce (if desired). Refrigerate several hours before serving.
To serve: ladle into individual bowls and garnish with cucumber, sour cream and hot sauce.