Peel and dice the onion. Clean the zucchini, cut three pieces into small pieces. Clean the soup vegetables and dice them. Sauté the onion, soup vegetables and zucchini pieces in 2 tablespoons of hot butter. Pour in 3/4 l water, bring to the boil and season. Cover and simmer for 10 minutes. In the meantime, grate the fourth zucchini. Cut the toast into cubes. Peel and slice the garlic. Rinse the basil briefly and finely chop except for a few leaves. Add the chopped basil to the soup and puree everything. Add the drained zucchini grated and cream and simmer for another 2 - 3 minutes. Toast the bread cubes and garlic in 1 tablespoon of hot butter (be careful not to burn the garlic!). Season the soup to taste and garnish with the bread cubes and the remaining basil leaves.