Peel onions and cut them into fine pieces. Sweat with olive oil in a saucepan, add the diced zucchini and let brown lightly. Add water and soup cubes and let simmer covered for half an hour.
Remove the saucepan from the heat and add the sour cream or whipped cream. Puree everything together with the blender and serve with the plucked basil and a little grated Parmesan.