Remove crust from bread. Cube 1 slice. Soak in 4 tablespoons lukewarm water and season with salt.
Blanch tomatoes, peel, remove seeds and chop.
Rinse peppers, remove white inner skin and seeds and finely dice. Peel cucumber, remove seeds and finely chop.
Peel garlic and finely chop.
Puree soaked bread, broth, tomato, garlic, tomato paste and juice with 2/3 each of the peppers and the cucumber. Add olive oil, lemon juice and sugar. Season with salt and cayenne pepper. Chill for about 1 hour in the refrigerator.
Cube remaining bread and saute until crispy in hot butter to make croutons. Remove gazpacho from refrigerator, pour into bowls and top with remaining diced vegetables and croutons.