Melt the butter in a large pan over medium heat. Add the onion and garlic and fry until translucent, about five minutes.
Add the fennel, leek, and potato to the pan. Fry for five minutes.
Pour the vegetable stock into the pan and bring it to a boil. Simmer until the fennel is completely soft, which can take up to 25 minutes.
Remove the pan from the heat, then use an immersion blender to purée the soup.
Pour in the cream and mix well. Season the soup with salt and pepper. Serve the creamy fennel soup warm, garnished with fresh dill and, if desired, smoked salmon. Enjoy!