How to Make It
Place fennel, oil, fennel seeds, and salt in a small saucepan. Bring to a simmer over low, and cook until fennel is soft and caramelized, about 30 minutes. Stir in 2 tablespoons lemon zest. Remove from heat, and let cool completely.
Set a medium bowl in an ice bath. Add crème fraîche and mascarpone cheese to prepared bowl, and whisk until smooth. Add remaining 1 tablespoon zest and lemon juice. Using a rubber spatula, fold smoked fish into crème fraîche mixture, and fold in fennel mixture. Transfer to a serving bowl, and smooth top. Cover and chill at least 1 hour and up to 3 days.
Let rillettes stand at room temperature 30 minutes before serving. Garnish with fennel fronds and piment d’Espelette. Serve with crackers and crudités.