Clean and wash the onions and cut into fine rings. Heat the butter in a sufficiently large saucepan and steam the onion rings in it. Add the soup greens and cook for about 2 minutes. Scatter the flour over it and let it sweat briefly. Remove the pan from the heat and fill with the vegetable stock, stir until smooth and bring to a boil. Add the cream and season the soup with herb salt, lemon juice and the grated lemon zest. Finally add the basil and puree everything with a hand blender or put in the blender.
Cut the bread into fine cubes and roast them together with the sunflower seeds in a non-fat frying pan until crispy. Sprinkle over the hot soup and serve.