How to Make It
In a small skillet, toast the fennel seeds over moderately low heat until golden, 3 minutes. Finely crush them in a mortar.
In a food processor, mix the fennel seeds with the mayonnaise, crème fraîche, garlic, fennel fronds, 2 teaspoons of the lemon zest and 1 tablespoon of the lemon juice; puree. Season with salt and black pepper.
In a bowl, mix the onion, olives, capers, crushed red pepper and the remaining 1 teaspoon of lemon zest and 2 tablespoons of lemon juice. Fold in the tuna and olive oil.
Spread the fennel mayo on the cut sides of the baguette. Arrange the arugula on the bottom baguette and top with the shaved fennel and the tuna. Layer the tomatoes, cucumber and anchovies on top. Close the baguette and cut into 4 sandwiches.