Grate the zucchetti through the rösti grater into a suitable bowl, pour the juice of one lemon over it.
Whisk together 2/3 of the zucchetti, yogurt and cream cheese. Thin to desired consistency with buttermilk. Season to taste and refrigerate until ready to use.
Cut tomatoes diagonally, remove stem and core. Cut into small cubes. Cut the basil into tender strips.
Before serving, mix the remaining zucchetti, diced tomatoes and basil strips into the soup.
Tip: The flesh of young, small zucchini is much more tender than that of larger ones.