Heat the olive oil in a stockpot over medium-high heat. Add the onion, celery, fennel, bell pepper, and garlic. Cook for 10 minutes, until very soft. Do not let the vegetables brown. Add the bouquet of herbs, tomatoes, and 2 1/2 cups water. Bring to a boil and cook for 15 minutes, or until the tomatoes have softened. Season with salt and pepper.
While the soup is cooking, fill a large pot with very cold water and ice cubes to make an ice water bath. When the soup is ready, place the stockpot in the ice water bath to cool. Remove the bouquet of herbs.
When the soup has cooled, transfer it to a blender and purée until smooth. Pass the purée through a fine-mesh strainer into a bowl. Add the Tabasco sauce and celery salt, and season to taste with salt and pepper. Set the soup aside.
Place the avocado in a small bowl and mash it with a fork until smooth. Stir in the lime zest and juice, red onion, jalapeño, bell pepper, and cilantro, and season with salt and pepper.
Ladle the chilled soup into 20 small glasses or bowls. Place a spoonful of the guacamole in the center of each one, and garnish with a basil leaf. Arrange on a platter, and serve cold.