Cut the onion into small cubes and fry in the olive oil. Meanwhile, cut the zucchini in half and slice, and dice the tomatoes. Steam these for about 5 - 10 minutes.
Pour in the broth and simmer the whole thing for another 10 minutes, until the zucchini are translucent. Then puree everything and season to taste, add the dash of cream and bring to the boil again briefly. Arrange on a soup plate and garnish with basil leaves.