How to Make It
In a microwave*#150;safe bowl, heat 2 tablespoons of the fish stock at high power. Crumble in the saffron and let cool. In a large bowl, soften the bread in the remaining 1/4 cup of fish stock.
In a food processor, puree the soaked bread, egg yolk, piquillo, garlic and lemon juice until smooth. With the machine on, slowly pour in 1/4 cup of the oil and the saffron fish stock. Slowly pour in the remaining 1/4 cup of oil. Season with cayenne and salt.
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.