Put tomatoes, scallions, garlic and most of the cucumber (retain a small amount for garnish) into the blender and blend until smooth. Pass through a fine sieve to remove most of the pulp.
Put the mixture back in the blender and slowly add the olive oil, lemon juice and sherry vinegar until combined before adding the basil and parsley. Be careful not to blend the herbs for too long as you want them to remain finely chopped, not puréed. Chill in the refrigerator.
To make the garnish, gently fry the bread in olive oil to make croutons. To serve, season gazpacho well before transferring to bowls and topping with the chopped vegetables and croutons.