Heat the oil in a large pot on a medium flame. Put the scallions and celery in, season with salt and stir for a few minutes. Put the garlic in and keep cooking for a 2-3 more minutes.
Pour the water in with the zucchini chunks. Cover the pot and raise heat to boil. Then lower and cook covered for an hour.
Put the whole mixture into a blender and blend to a smooth texture. Garnish with parsley and black pepper.