Wash and clean the fennel. Halve the tuber and cut into thin slices. Peel and finely dice onions and garlic. Roughly dice the sausage and leave it crispy in a pot without fat. Add onions, garlic, fennel and tomato paste and fry.
Deglaze with broth, bring to the boil and simmer for about 3 minutes.
Add the tomatoes and bring to the boil. Drain beans, rinse and add to tomatoes. Let the soup simmer gently for about 15 minutes. Wash basil, shake dry, pluck leaves from the stalks, put some leaves aside, cut remaining leaves into fine strips.
Season soup with salt, pepper and basil strips. Arrange soup in bowls and garnish with basil leaves.