Peel and finely chop the onion. Peel garlic and chop finely. Clean and wash the zucchini, dice 3/4 of the zucchini. Cut the rest of the zucchini into fine slices and keep covered in a cool place. Peel, wash and finely dice the potatoes. Heat oil in a high pot, fry onions in it until translucent. Add garlic, potatoes and diced zucchini, fry briefly.
Deglaze with broth, bring to the boil, cover and simmer over medium heat for about 15 minutes. Wash lemon balm and chervil, dab dry and add herbs 5 minutes before end of cooking time. Then remove from the heat and puree with a blender. Season to taste with salt, pepper, sugar and lemon juice. Pour into a bowl and put in a cool place for about 1 1/2 hours. Then stir in buttermilk. Serve in deep plates with zucchini slices and garnish with lemon balm and chervil as desired