In a large pot, heat the oil over medium heat. Add the fennel and cook, stirring, until it starts to soften, about 5 minutes. Add the celery, onion, and garlic. Cook, stirring, 5 minutes longer to allow the flavors to blend together. Carefully add the Pernod. (Caution! It could flame up.) Add the dill, celery seeds, and pepper. Add the vegetable stock and tomatoes, and bring the mixture to a boil. Reduce the heat to a simmer and let simmer for 40 minutes.

Remove the pot from the heat. Process the soup, in batches, in a blender or food processor fitted with the metal chopping blade until smooth. Add the butter, if desired, and stir until melted. Taste and check the seasoning, adding salt if needed.