How to Make It
Combine all of the ingredients in a saucepan. Cover, bring to a simmer and cook until the mussels open, about 3 minutes. Strain the broth and reserve the mussels.
In a large skillet, melt the butter. Add the fennel and cook over moderately low heat, stirring occasionally, until tender and golden, about 15 minutes. Add the leeks and cook, stirring occasionally, until tender and
the fennel is lightly caramelized, about 8 minutes more. Remove from the heat.
In a pot, melt the butter. Add the shallots and cook over moderate heat until softened, 2 minutes. Add the mussels and Pernod and cook, stirring, for 1 minute. Add the mussel broth and the fennel mixture. Cover and cook until the mussels open, about 5 minutes; using a slotted spoon, transfer the mussels to a large bowl as they open.
Bring the broth to a simmer. Whisk in the crème fraîche and season with salt. Add all of the mussels and cook until hot. Transfer the mussels and broth to bowls; garnish with tarragon. Serve with toasted baguettes.