Prepare the Mussels
In a large pot, heat the oil. Add the diced fennel, onion and garlic and cook over moderately high heat, stirring, until softened, 7 minutes. Add the red pepper and cook for 30 seconds. Add the Pernod and boil over high heat for 2 minutes. Add the fish stock and bring to a boil. Add the mussels, cover and cook, shaking the pot, until they open, 5 minutes. Pile the mussels in a bowl; discard any that don't open. Stir the butter into the broth, add the fennel fronds and parsley; season with salt. Pour the broth over the mussels and serve with the rouille.