Pat dry the fish fillet, cut into slices and drizzle with lemon juice. Wash the fennel, cut off the greens and set aside. Clean the tubers, quarter them and dice them without the middle stalk. Peel the potatoes and cut into small cubes. Peel and chop the onions. Mash the fennel seeds.
Heat the butter, sauté the onions, fennel and potatoes for 5 minutes. Stir in the fennel seeds. Pour in the stock, bring to the boil, cover and cook for 10 minutes. Take out a few pieces of vegetables, puree the soup, season with salt and pepper. Stir in the cream, add the vegetables.
Add the pikeperch and let it steep for 7 minutes over a low heat. Season the soup to taste. Chop the fennel greens and sprinkle over them. A nice fresh ciabatta also tastes good.