Peel and finely dice the onion and garlic. Clean zucchini, peel potatoes. Wash both and cut into large pieces.
Heat 2 tablespoons of oil in a large pot. Sauté onion and garlic, add zucchini and potatoes and cook for 3-4 minutes. Approx. 3 ⁄ Stir in 4 l water and stock. Bring everything to the boil, cover and simmer for about 15 minutes.
Chop the walnuts and roast them in a large coated pan without fat. Remove. Heat 2 tablespoons of oil in the pan and fry the slices of bread in it from both sides until golden brown. Season with salt and spread the walnuts on top.
Finely puree the soup with the cutting stick. Stir in cream and bring everything to the boil briefly. Season with salt, pepper, lemon juice and 1 pinch of sugar.
Sprinkle soup with pumpkin seed oil and serve with nut crostini.