Cook the leeks in the butter in a saucepan over medium heat until the leeks wilt. add the fennel bulb and chicken broth. Cover and simmer over low heat for 30 minutes. Stir in the fennel flowers, green seeds, or pollen.

Puree the soup in batches in a blender until very smooth. Be very careful when doing this. Fill the blender no more than half full so that the hot soup does not splash out, and pulse it in quick spurts before switching it on continuously. Pour it back into the saucepan and reheat it until it simmers. Taste the soup, season with pepper, and add salt if needed. Serve hot or chilled, garnished with a small pinch of fennel flowers, or chopped fresh seeds.